Applied Anatomy


This chapter will address general anatomical principles as well as cover detailed anatomy relevant to the obstetrician and gynaecologist and the Membership of Royal College of Obstetricians and Gynaecologists (MRCOG) exams. Particular attention is given to how this knowledge should be applied clinically, and readers are advised to refer to more detailed comprehensive anatomy books to supplement this applied anatomy approach.

Body Tissues and Cells

These are composed of four elements:

  • Epithelium

  • Connective tissue

  • Muscle

  • Nerve

Epithelium can be simple or stratified:

  • 1.

    Simple means it is one layer thick and this is seen with absorptive or secretory surfaces:

    • Simple squamous – flat cells (e.g. endothelium)

    • Simple cuboidal – collecting ducts

    • Simple columnar – gut lining/fallopian tubes

  • 2.

    Stratified means it has multiple layers and this affords protection:

    • Stratified squamous – and if this is also keratinised it comprises skin – vagina

    • Stratified cuboidal – (2 to 3 cells thick) (e.g. excretory duct)

    • Stratified transitional – cuboidal cells right up to surface (i.e. surface cells remain large; e.g. urinary epithelium)

Other histology details are addressed in the relevant sections of the text.

The Nervous System

The central nervous system (CNS) comprises the brain and the spinal cord, while the peripheral nervous system includes the cranial and spinal nerves. Both central and peripheral systems have somatic (aware/voluntary) and autonomic (unaware/involuntary) components.

The Somatic Nervous System

This both transmits sensory information (afferent pathways) and innervates skeletal muscle (efferent pathways). The sensory cells are derived from the neural crest and are bipolar with their cell bodies lying in the dorsal root ganglia ( Note there is no synapse in dorsal root ganglia), while the motor cells grow out in the ventral root from the neural tube (i.e. single myelinated cells with no synapses before their end organs). Somatic nerves do not cross the midline ( Fig. 5.1 ).

Fig. 5.1

Diagrammatic representation of a transected spinal cord showing the somatic and autonomic neurone pathways and the main tracts running within the cord.

Spinal nerves consist of:

  • Posterior primary rami sequentially supplying erector spinae and overlying skin

  • Anterior primary rami supply the rest of the body’s muscles and skin and are often involved in forming nerve plexuses before branching and joining together for more distal distribution (cervical, brachial, lumbar and sacral plexuses)

Autonomic nerves often ‘hitch a ride’ on these nerves (as they do on blood vessels).

The Autonomic Nervous System

Unlike the somatic nervous system, this is involuntary and regulates the body’s internal environment. It has the distinctive feature of comprising two neurones in its motor pathway which synapse outside the CNS: one neurone grows out from the CNS and is myelinated (preganglionic) while the postganglionic neurone (derived from neural crest cells) is unmyelinated. Two components (sympathetic and parasympathetic) form the autonomic system and tend to oppose each other to maintain internal homeostasis. The sympathetic nervous system originates from the thoracolumbar regions and the ganglia form a chain bilaterally down each side of the vertebral column, while the parasympathetic nerves have craniosacral outlets and their ganglia are situated distally near their target organs. These differences together with pharmacological features are illustrated in Fig. 5.2 .

Fig. 5.2

Diagrammatic representation of the neurone arrangements and neurotransmitters of the somatic and autonomic nervous systems. CNS , Central nervous system.

In addition to these efferent autonomic motor neurones, there are afferent fibres which are conveyed via the sympathetic and parasympathetic nerves, but they are independent of them and do not relay in the ganglia. Like other sensory fibres, their cell bodies lie in the dorsal root ganglia from where they ascend centrally to the hypothalamus and thence to the orbital and frontal gyri of the cerebral cortex (see Fig. 5.1 ).

Clinical Application

In normal circumstances, we are unaware of autonomic afferent impulses but if sufficiently strong they can cause the sensation of visceral pain (intestinal colic, uterine pain, etc.) which can also produce referred pain in the dermatome of the relevant segmental supply (e.g. cervix S2 and S3, ovary T10 and T11, body of the uterus, lower thoracic and upper lumbar roots). Dermatomes are shown in Fig. 5.3 .

Fig. 5.3

An approximate pattern of anterior and posterior dermatomes.

Sympathetic (Thoracolumbar) Nervous System

These cells are derived from the lateral horn of T1–L2 but the preganglionic fibres travel up and down to form a chain of ganglia extending from the cervical to the coccygeal region (i.e. the root value of the autonomic component may be different from the spinal component with which it emerges). Postganglionic neurones then form sympathetic nerve plexuses, the main ones being:

  • Cardiac plexus (below the aortic arch)

  • Pulmonary plexus (at the root of the lungs)

  • Coeliac plexus (on the coeliac axis and around the origin of the superior mesenteric artery)

  • Superior hypogastric plexus (anterior to the aortic bifurcation)

  • Inferior hypogastric plexus (lateral to the rectum, cervix and vaginal fornix)

The peripheral distribution of the sympathetic fibres includes branches for somatic distribution which travel with each spinal nerve to supply the corresponding segmental skin, and the visceral distribution which tends to reach its end organ by means of the arterial pathways.

Approximate segmental supplies:

  • T1–2 head and neck

  • T1–4 thoracic viscera

  • T2–5 upper limb

  • T4–L2 abdominal viscera

  • T10–L2 pelvic viscera

  • T11–L2 lower limb

Note : Thoracic, lumbar and sacral splanchnic nerves emerge from the sympathetic plexuses while the p elvic splanchnics, in contrast, are parasympathetic (S23 – nervi erigentes). These parasympathetic preganglionic fibres join the sympathetic fibres (from the inferior hypogastric plexus) for distribution within the pelvis and are described in more detail later.

Sympathetic Effects

These are essentially of fight and flight:

  • Vasoconstrictor (except to coronary arteries which it dilates)

  • Increases the heart rate

  • Dilates the bronchial tree

  • Relaxes the detrusor muscle

  • Contracts smooth muscle sphincters

  • Dilates the eye (by relaxing the ciliary muscle)

  • Relaxes the small intestine

The Adrenal Medulla

This is derived from neural crest cells and comprises approximately 10% of the adrenal gland (the remainder being the cortex). The myelinated preganglionic sympathetic fibres from the splanchnic nerves travel via the coeliac plexus and synapse directly with the medullary (chromaffin) cells which secrete catecholamines.

Parasympathetic (Craniosacral) Nervous System

Unlike the sympathetic, the parasympathetic system has no somatic distribution and is purely visceral. Four cranial nerves and two sacral roots are involved.

Cranial: III, VII, IX, X

The vagus is particularly important, travelling widely. It forms a plexus round the oesophagus and the fibres from each side mix and thence continue as anterior and posterior nerves. The vagus contributes to the:

  • Cardiac, pulmonary and oesophageal plexuses

  • Stomach and liver (via the anterior vagus)

  • Small and large intestine as far as the splenic flexure (travelling via the superior mesenteric artery)

Sacral: S2 and S3

Preganglionic cell bodies lie in the lateral horn of the grey matter in the spinal cord from where they pass out as nervi erigentes to intermingle with the inferior hypogastric plexus to supply the:

  • Gut beyond the splenic flexure (travel via the inferior mesenteric artery)

  • Bladder

  • Genital organs

  • Pelvic blood vessels

Parasympathetic Effects

  • Decreases the heart rate

  • Bronchoconstrictor

  • Increases glandular secretions

  • Increases peristalsis

  • Stimulates detrusor contractions

  • Relaxes sphincters

The Spinal Cord and Meninges

This extension of the CNS begins in the medulla oblongata at the foramen magnum and ends at L1/L2. The nerve roots that continue after the spinal cord has terminated at the conus medullaris comprise the cauda equina, while the filum terminale (the extension of pia mater) inserts into the coccyx. As the length of the spinal cord is shorter than the vertebral column, nerve roots arise at increasingly higher levels than their corresponding vertebrae and travel increasingly longer distances within the vertebral column before exiting from their respective vertebral foramina.

The membranes of the cord are termed the meninges and they comprise neuroepithelium of which there are three layers. From outside inwards these are:

  • 1.

    Dura mater (under which lies the subdural space)

  • 2.

    Arachnoid mater (under this is the subarachnoid space containing cerebrospinal fluid)

  • 3.

    Pia mater

Both pia and arachnoid are continued out along the spinal nerve roots, while the dura forms a tough sheath for the cord ending at S2, and it extends out over each nerve root blending with its sheath.

The epidural (extradural) space lies between the dura mater and the spinal canal and is filled with fat and vessels (lymphatic and blood).

The spinal cord has neuronal cell bodies in its grey matter, and its external white matter comprises axonal tracts (see Fig. 5.1 ).

Afferent neurones include those for:

  • Touch and vibration – cell bodies in dorsal root ganglia, tracts in posterior columns

  • Pain and temperature – cell bodies in contralateral posterior horn, axons in lateral and anterior spinothalamic tracts

  • Proprioception – axons in lateral spinocerebellar tracts

Efferent neurones are motor and pass along the:

  • Lateral cerebrospinal (or corticospinal) tract. These neurones originate in the motor cortex but the fibres cross before descending in what is also referred to as the crossed pyramidal tract.

  • Anterior cerebrospinal (or direct pyramidal) tract. These neurones are uncrossed.

Spinal Nerve Roots and their Plexuses

Each pair of spinal nerves emerges from the vertebral column as illustrated in Fig. 5.1 , and branches proceed to supply the skin in a pattern which can be mapped out diagrammatically (see Fig. 5.3 ).

Clinical Application

Pain can be referred to the dermatome which is supplied by the same nerve root as the area in question. Some spinal nerves merge and re-divide with other nerve roots before proceeding. This produces nerve plexuses and these occur in the cervical, brachial, lumbar and sacral regions. Although the cervical/brachial plexus can be relevant in situations of obstetric trauma to the neonate (in the clinical situations of shoulder dystocia), detailed knowledge of it is beyond the remit of this chapter. The relevant clinical message is to respect the fetal neck and avoid undue traction on it (which can stretch and damage the nerve roots).

The lumbar and sacral plexuses are described in the relevant regional anatomy sections (pp. 22 and 24).

Anatomy of the Brain

The brain develops from the neural tube and its cavity persists in the three resulting components:

  • The forebrain

    • the cerebral hemispheres, each with their lateral ventricle

    • the deeper diencephalon surrounding the third ventricle

  • The midbrain

    • connects the forebrain to the hind brain

    • the aqueduct (of Sylvius) runs through it

  • The hindbrain

    • pons, medulla oblongata and cerebellum

    • the fourth ventricle

  • The midbrain, pons and medulla comprise the brain stem

The Thalami

The two thalami lie laterally in the diencephalon forming the lateral walls of the third ventricle. The internal capsule lies laterally, separating them from the basal ganglia. The thalamus is sensory in function and relays impulses on to the cerebral cortex via the internal capsule. It also connects to the hypothalamus.

The Hypothalamus

The hypothalamus is also in the diencephalon forming the floor of the third ventricle and is concerned with the autonomic nervous system. It contains many cell types, in particular the supraoptic and paraventricular nuclei whose axons connect it to the posterior lobe of the pituitary via the pituitary stalk. It also connects with the basal nuclei caudally and via long axons to the sympathetic and parasympathetic cells in the lateral horns of the spinal cord.

The Pineal Gland

The pineal gland lies posterior to the thalamus at the posterior end of the third ventricle and is innervated by the sympathetic nervous system. It is most active at night, produces melatonin and tends to have an inhibitory effect on other endocrine glands and gonads. It calcifies with age and may be visible on a skull x-ray after the age of 40 years.

The Pituitary Gland

The pituitary gland is composed of two parts; both are derived from ectodermal tissue but of different origins:

  • The small posterior pituitary is derived from a downgrowth of ectodermal neural plate and these neurones have their cell stations in the hypothalamus. These neurosecretory cells produce oxytocin and antidiuretic hormone (adh)

  • The larger anterior pituitary (pars tuberalis) forms from Rathke’s pouch growing up from the roof of the mouth and consists of glandular cells:

    • chromophobes – account for 50% of the anterior pituitary

    • eosinophilic/acidophilic cells produce growth hormone (GH) and prolactin

    • basophilic cells produce adrenocorticotrophic hormones (ACTH), follicle stimulating hormones (FSH), luteinizing hormones (LH) and thyroid stimulating hormones (TSH)

This gland occupies the pituitary fossa with:

  • The diaphragma sellae and optic chiasma above

  • The cavernous sinuses laterally

  • The body of the sphenoid below

Clinical Application

Pituitary tumours (including prolactinomas) can grow upwards to press on the medial sides of the optic nerves in the lower anterior part of the optic chiasma causing temporal hemianopia (tunnel vision).

The Lymphatic System

Lymphatic Vessels

The extracellular tissues of the body are constantly gaining fluid and debris (from capillary leakage, cell death, etc.) and the function of the lymphatics is to remove this and return it to the venous circulation. The lymphatic capillaries have the same basic structure as vascular capillaries but their distribution is not uniform throughout the body. The lymphatics in the limbs tend to be superficial, while those of the viscera tend to drain via channels on the posterior abdominal and thoracic walls.

The lymphatic vessels return the lymph to the venous system via two main channels:

  • The right lymphatic duct drains the right thorax, upper limb, head and neck

  • The thoracic duct drains all lymph from the lower half of the body

The pre- and para-aortic lymphatics drain into the cisterna chyli which is an elongated sac-like vessel that lies over the body of L1 and L2 behind the inferior vena cava (IVC) and between the aorta and the azygous vein. It becomes the thoracic duct as it ascends through the diaphragm at the level of T12. It starts on the right side of the oesophagus, but as it ascends through the thorax the thoracic duct passes behind the oesophagus (at T5) to reach its left side, then superiorly it passes over the left subclavian artery and the dome of the left pleura to drain into the confluence of the left subclavian with the left internal jugular veins.

Lymphatics, like blood vessels (and unlike somatic nerves), can cross the midline, but in contrast they pass to and from lymph nodes (afferent and efferent lymphatics) and they comprise an anastomosing low-pressure system.

Lymphatic Tissue

These comprise concentrations of lymphocytes and occur in mucosal and submucosal collections in the gut (e.g. Peyer’s patches in the ileum) as well as in the thymus, the spleen and lymph nodes themselves.

The anatomical clinical importance of this system relates to the drainage patterns of each group of nodes, which is summarised in Table 5.1 , but also described for the individual organs in their relevant regional anatomy sections.

Table 5.1

Lymphatic Drainage Patterns

Lymph Node Group Location Tissues/Structures Drained
Superficial inguinal nodes Longitudinally along the great saphenous vein and horizontally distal to the inguinal ligament

  • Anterior abdominal wall (below umbilicus)

  • Upper part of uterus and round ligament

  • Lower third of vagina, vulva, perineum and anus

  • Superficial part of leg and buttock

Deep inguinal lymph nodes Lie medial to the femoral vein

  • The superficial inguinal nodes

  • Deep part of leg

  • Clitoris

Deep femoral lymph node of Cloquet Lies in the femoral canal Vulva
External iliac nodes Along the external iliac arteries

  • Deep inguinal lymph nodes

  • Bladder

  • Lower uterus and cervix

Internal iliac nodes Along the internal iliac arteries

  • Urethra and deep perineum

  • Cervix and upper two-thirds of vagina

  • Lower rectum

Common iliac nodes Along the common iliac arteries

  • Internal and external iliac nodes

  • Abdominal part of the ureter

  • Fallopian tubes and upper uterus

Obturator nodes Along the obturator artery Cervix
Para-aortic nodes Lie alongside the aorta near the origins of the paired arterial branches

  • Common iliac nodes

  • Posterior abdominal wall

  • Lumbar region

  • Kidneys and ovaries

Pre-aortic nodes Anterior to the aorta around the origin of coeliac, superior and inferior mesenteric arteries Pelvis and abdomen corresponding to ventral aortic arterial branches

The Vascular System

Fetal Circulation and Changes after Birth

Oxygenated Blood

  • The ductus venosus bypasses the liver taking oxygenated blood from the left branch of the portal vein (from the umbilical vein) to the IVC

  • This flows into the right atrium and is directed towards the foramen ovale passing through into the left atrium and thence out to supply the head and neck.

Deoxygenated Blood

  • Flows back from the superior vena cava (SVC) and is directed through the tricuspid valve to the right ventricle

  • The ductus arteriosus bypasses the lungs taking blood from the left branch of the pulmonary trunk to the aorta distal to its three main primary branches

  • The blood in the descending aorta then passes out to the placenta via the umbilical arteries which branch off from the internal iliac arteries

Changes at and After Birth

  • The pressure changes due to inflation of the lungs and the increased flow through the pulmonary arteries close the foramen ovale

  • The ductus arteriosus muscular wall contracts and closes, and is effectively obliterated within 2 months, becoming the ligamentum arteriosum

  • The ductus venosus becomes the ligamentum venosum (passing round the caudate lobe of the liver)

  • The intra-abdominal umbilical vein becomes the ligamentum teres

  • The umbilical arteries become obliterated and form the medial umbilical ligaments (not to be confused with the median umbilical ligament which is the obliterated remains of the urachus)

The Arterial System

The Aorta

The aorta ( Fig. 5.4 ) enters the abdomen behind the diaphragm between its crura at T12 and descends to divide into the common iliac arteries at L4. It has three ventral branches which give rise to the portal circulation, while the other branches are systemic.

Fig. 5.4

The abdominal aorta and its branches.

Three Ventral Branches.

  • The coeliac artery (axis/trunk) is very short (1 cm long) arising at level L1

  • The superior mesenteric artery arises at level L2

  • The inferior mesenteric artery arises at level L3

Three Terminal Branches.

  • The right and left common iliac arteries arise at level L4

  • The median sacral artery continues over L5

Four Pairs of Branches.

  • Phrenic arteries

  • Suprarenal arteries

  • Renal arteries

  • Gonadal arteries

Four Lateral Pairs.

  • The four lumbar segmental arteries.

The Common Iliac Arteries

The common iliac arteries diverge from in front of the fourth lumbar vertebra and then divide into internal and external iliac arteries in front of the sacroiliac joint.

The external iliac artery is essentially involved in the blood supply to the leg (becoming the femoral artery when it passes behind the inguinal ligament), but it gives two important branches off just above the inguinal ligament: the inferior epigastric and the deep circumflex iliac arteries.

The internal iliac artery divides into anterior and posterior branches to supply the pelvis and buttock, respectively. Details of these vessels are given in the section on the pelvis.

Details of individual vessels and their relations are given in the relevant regional anatomy sections.

The Venous System

This is a relatively low-pressure valved system for draining blood back to the heart. Flow fluctuates with the arterial pulse while muscle pumps further encourage flow in the limbs and inspiration increases flow in the IVC and SVC centrally. Excepting the portal circulation, veins generally follow the pattern and path of arteries and have sympathetic innervation.

The Inferior Vena CAVA

The common iliac veins join to form the IVC ( Fig. 5.5 ) behind the right external iliac artery at L5. The IVC ascends through the abdomen on the right of the aorta piercing the central tendon of the diaphragm at T8. It receives:

  • Segmental lumbar veins

  • The right gonadal vein (the left gonadal vein drains into left renal vein)

  • The renal and suprarenal veins

  • The hepatic veins

  • The inferior phrenic veins

Fig. 5.5

The inferior vena cava and its tributaries.

Collateral Venous Drainage Pathways

There is an extensive network of potential collateral circulations which open when thrombosis of the IVC occurs.

Superficial venous channels which can eventually drain to the SVC are:

  • Epigastric

  • Circumflex iliac

  • Superficial epigastric and lateral thoracic (via thoracoepigastric vein)

  • Internal thoracic

  • Posterior intercostals

  • External pudendal

  • Lumbovertebral

Deep channels which provide deep anastomoses are:

  • Azygous

  • Hemiazygos

  • Lumbar

The vertebral venous plexus also provides effective collateral circulation between IVC and SVC.

Clinical Application

This collateral circulation is so efficient that, even when there is substantial obstruction to venous flow by a large deep vein thrombosis in the iliac vessels, there can be an absence of clinical symptoms or signs.

The Portal Venous Drainage and Portosystemic Venous Anastomoses

The portal venous system drains blood to the liver from the abdominal part of the alimentary canal (except the anus), the spleen, pancreas and gall bladder. The superior and inferior mesenteric veins join the splenic vein behind the pancreas to form the portal vein which carries blood to the liver, which in turn is drained by the hepatic veins which pass into the IVC. This pathway may be obstructed causing portal hypertension and then collaterals open up between the portal and the systemic venous systems:

  • Lower oesophagus – tributaries of: left gastric with hemiazygos/azygous

  • Anal wall – superior rectal with middle and inferior rectal

  • Caput medusa – tributary from left branch of portal vein (paraumbilical) with epigastrics

  • Retroperitoneal veins of abdominal wall with veins of the ascending colon and the bare area of the liver

  • Very rarely a patent ductus venosus

Vertebral Column

Venous drainage from both the internal and the external vertebral plexus drain to regional segmental veins providing potential communication with systems which also drain segmentally. This is a largely valveless system and therefore the spread of malignancy is possible (especially likely from breast, uterus, prostate and thyroid):

  • Pelvic viscera via the lateral sacral vessels

  • Abdomen via the lumbar veins

  • Breast via the posterior intercostals

  • Neck via the vertebral vein

The Musculoskeletal System

Types of Joint

  • Fibrous (bone/fibrous tissue/bone) (e.g. skull sutures although these ossify in later life)

  • Cartilaginous:

    • primary (bone/hyaline cartilage/bone) (e.g. epiphyses or costochondral junctions)

    • secondary (bone/hyaline cartilage/fibrocartilage/hyaline cartilage/bone) – these only occur in the midline (e.g. pubic symphysis, intervertebral joints)

  • Synovial joints that allow movement (e.g. hip joint). The sacroiliac joint is also a synovial joint but atypical in that the movement allowed is extremely limited

The Vertebral Column

The vertebral column has 33 vertebrae (7 cervical, 12 thoracic, 5 lumbar, 5 sacral and 4 coccygeal). The five sacral vertebrae are fused to form the sacrum, and the coccygeal components can be variably fused.

There are 31 pairs of spinal nerves whose nerve roots travel variable distances within the vertebral column to exit the spine by passing across the disc of the vertebra above (therefore problems with, e.g. L4 disc will affect L5 nerve root).

The Pelvis

The bony pelvis comprises the sacrum and the os innominatum.

  • The sacrum is composed of five fused vertebrae (with four sacral foramina). It articulates with the fifth lumbar vertebra above, the coccyx below and the ilium laterally.

  • The os innominatum is made up of three bones: ilium, pubis and ischium, which are joined by cartilage in the young, but by bone in adulthood. They meet in a Y-shaped junction in the acetabulum to which they all contribute.

Clinical Application

Movement at the pelvic joints is minimal in the non-pregnant state, but there is considerable joint relaxation during pregnancy. In some women, instability can occur with sacroiliitis or pubic symphysis dysfunction which can be extremely debilitating. Limiting abduction of the legs in these conditions is crucial in preventing further deterioration or even permanent instability, and pain-free abduction distances should be measured (knee to knee) and recorded prior to labour so that nursing of the woman (when pain-free with an epidural) does not silently cause more damage.

Obstetric Pelvic Definitions and Dimensions

The pelvic inlet is oval being widest transversely, the pelvic mid-cavity is circular, while the outlet is oval being widest anteroposteriorly. Normally, the fetal head enters the pelvis transversely due to the shape of the inlet and subsequent rotation of the fetal head during the descent through the pelvis in labour takes advantage of the bony dimensions, but the rotation itself is caused by the muscular pelvic gutter ( Table 5.2 ).

Table 5.2

Approximate Pelvic Obstetric Dimensions (cm)

Transverse Oblique Anteroposterior
Inlet 13 11 11
Mid-pelvis 12 12 12
Outlet 10.5 11.5 12.5

The Pelvic Inlet

The pelvic inlet is oval shaped and is widest from side to side. It divides the bony pelvis into the false pelvis above (made up mainly of the ala of the ilium on each side which forms the lower lateral portion of the abdomen), and the true pelvis below (the pelvic cavity). The boundaries of the pelvic inlet include:

  • The promontory of the sacrum

  • The arcuate line of the ilium

  • The iliopubic eminence

  • The pectineal line

  • The pubic crest

  • The symphysis pubis

The Pelvic Outlet

The pelvic outlet is widest from front to back and lies between:

  • The lower border of the symphysis pubis anteriorly

  • The ischial tuberosities laterally

  • The tip of the last sacral vertebra posteriorly

The true obstetric conjugate extends from the sacral promontory to the upper border of the pubic symphysis. The diagonal conjugate extends from the sacral promontory to the lower border of the pubic symphysis. The important landmarks of the pelvis are indicated in Figs. 5.6 and 5.7 .

Fig. 5.6

Important landmarks of the pelvis.

Fig. 5.7

Lateral view of the pelvis showing the obstetric conjugates.

The Male and Female Pelvis

General differences in structure between the male and female pelvis relate to the heavier thick-set skeleton of the male, with more obvious and well-marked muscle attachments and larger joint surfaces compared with the female, but there are also notable sex differences ( Table 5.3 ).

Table 5.3

Differences Between the Male and Female Pelvis

Sex Differences Female Male
Sacral curve

  • Short, wide and flat

  • Curved in the lower part

  • Long and narrow

  • General curve

Articular surfaces of the sacrum

  • Laterally with two sacral bodies

  • Superiorly with L5: oval and occupies one-third of alar surface

  • Laterally with three sacral bodies

  • Superiorly with L5 and occupies half of the alar surface

Pelvic inlet Oval Heart shaped
Pelvic canal Short and almost cylindrical Long and tapered
Pelvic outlet Comparatively large Comparatively small
Subpubic angle Approx 80–90 degrees 50–60 degrees
Obturator foramen Triangular Oval

Variations in Pelvic Shape

( Fig. 5.8 )

Fig. 5.8

Diagrammatic representation of different pelvic shapes.

  • Gynaecoid – normal female

  • Android – normal male

  • Anthropoid – the pelvic brim is longer anteroposteriorly than transversely

  • Platypelloid – the pelvic brim is much wider transversely and foreshortened anteroposteriorly

  • Rachitic pelvis – typical of rickets and the result of vitamin D deficiency. The sacral promontory projects forwards reducing the anteroposterior diameter

  • The contracted pelvis – can be symmetrical associated with a small stature, or asymmetrical due to a variety of disease processes

  • A narrow (gothic) subpubic arch foreshortens the effective pelvic outlet because the narrow anterior triangle (the waste space of Morrison) cannot accommodate the fetal head. In such circumstances, more space is required posteriorly to enable vaginal delivery ( Fig. 5.9 ).

    Fig. 5.9

    Illustration of the effect of a narrow subpubic arch and the waste space of Morrison.

Ligaments of the Pelvis

The vertebropelvic ligaments (see Figs. 5.6 and 5.7 ):

  • Iliolumbar – this V-shaped ligament extends from the transverse process of L5 to the iliac crest above, and the ventral portion of the sacroiliac ligament below (lumbosacral ligament)

  • Sacrospinous ligament runs from the lower lateral aspect of the sacrum and the upper lateral aspect of the coccyx to insert into the ischial spine

  • Sacrotuberous ligament is extremely strong opposing the forward tilting of the sacral promontory. It also originates from the lower lateral aspect of the sacrum and the upper lateral aspect of the coccyx inserting into the inner aspect of the ischial tuberosity.

The sacrospinous and sacrotuberous ligaments convert the greater and lesser sciatic notches into foramina (see Fig. 5.7 ).

The Fetal Skull

The skull base develops in cartilage, the vault in membrane. The fetal cranium consists of two frontal bones, two parietal bones and one occipital bone. These are separated by sutures and fontanelles and provide landmarks for defining the presentation of the fetal head in labour:

  • Occiput describes the area behind the posterior fontanelle

  • The vertex describes the parietal eminences between anterior and posterior fontanelles

  • The bregma is the area around the anterior fontanelle

  • The sinciput is the area in front of the anterior fontanelle which is further divided into brow and face (above and below the root of the nose).

The presenting diameter of the fetal skull varies according to its presentation:

  • Occipital and face presentations have the smallest diameters (suboccipitobregmatic and submentobregmatic, respectively) both being of the order of 9.5 cm

  • Vertex is most common with the occipitofrontal diameter of 11.5 cm

  • Brow is the largest with the mentovertical diameter of 13 cm.

Moulding during labour slides the parietal bones under each other and the occipital and frontal bones under the parietal bones and can reduce dimensions by 1 to 1.5 cm ( Fig. 5.10 ).

Fig. 5.10

Essential landmarks of the fetal skull: (A) from above and (B) lateral view.

Relevant Regional Anatomy of the Thorax

Surface Anatomy

Knowledge of the surface anatomy of the chest can be extremely valuable clinically:

  • The angle of Louis , which is the ridge produced by the manubriosternal joint, lies at the level of thoracic vertebra T4, but more useful is the site of the second costochondral junction marking the second rib from which subsequent intercostal spaces can be defined. These features also mark the upper limit of the surface markings of the heart.

  • The 4th intercostal space marks the dome of the diaphragm and the uppermost edge of the liver.


Ribs generate a negative pressure for respiration (−5 to −15 mmHg)

  • True ribs (ribs 1 to 7) articulate with the sternum

  • False ribs (8 to 10) – their costal cartilages articulate with the rib above

  • Floating ribs (11 and 12) have muscle attachments only.

The intercostal muscles which comprise three layers – external, internal and innermost – run between neighbouring ribs. The neurovascular bundles run along the lower inside border of each rib (i.e. superiorly in each intercostal space) between the internal and innermost intercostal muscles.

Clinical Application

  • When aspirating or inserting a chest drain, the position of the neurovascular bundle should be remembered and access should be achieved by running the needle or drain over the rib rather than under it. The fifth intercostal space in the mid-axillary line is usually used, but in pregnancy it is best to go up one space to allow for the raised diaphragm

  • The higher level of the diaphragm in pregnancy is also relevant in situations of trauma to the chest which is more likely to involve intra-abdominal organs

  • The parietal pleura is innervated segmentally from the intercostal nerves and therefore when inflamed produces pain which is referred to the cutaneous distribution of that nerve. Thus anterior abdominal wall pain can arise from pleural irritation mimicking an abdominal event.

The Diaphragm

This is a musculotendinous structure which separates the thorax from the abdomen. It arises from:

  • The xiphisternum

  • The lower six ribs and their costal cartilages

  • The medial and lateral arcuate ligaments

  • The first three lumbar vertebrae on the right/first two on the left (right and left crus) and fuses into a trifoliate central tendon below the pericardium.

The motor nerve supply is from the phrenic nerve (C3-5), and sensory supply is from the lower six intercostal nerves. The blood supply comes from the lower intercostal arteries superiorly, and the phrenic arteries (branches of aorta) inferiorly.

The three main openings in the diaphragm and their vertebral levels are as follows:

  • 1.

    The aortic opening at the level of T12 transmits the aorta with the thoracic duct and the azygous vein (from left to right).

  • 2.

    The oesophageal opening which passes through the right crus of the diaphragm at the level of T10, and also transmits the left gastric artery and both vagi.

  • 3.

    The IVC runs through the central tendon at the level of T8 together with the right phrenic nerve.

Other structures which penetrate the diaphragm include the greater and lesser splanchnic nerves and the sympathetic chain.

The Abdomen

Surface Anatomy

  • The transpyloric plane is an important landmark because of its anatomical relationships. It lies a patient hand-breadth below their xiphoid and is at the level of the first lumbar vertebra and the ninth costal cartilage and marks the termination of the spinal cord. Structures in this plane include the:

    • Pylorus of stomach

    • Duodenojejunal flexure

    • Fundus of the gall bladder

    • Renal hila

    • Neck of pancreas.

  • The subcostal plane joins the lowest costal margins on both sides and marks the tenth rib and the level of the third lumbar vertebra.

  • The plane of the iliac crests marks the bifurcation of the abdominal aorta at the level of the fourth lumbar vertebra.

  • The umbilicus is an inconsistent landmark, but in the slim adult lies at the lower part of the third lumbar vertebra, the third part of the duodenum and the origin of the inferior mesenteric artery.

  • McBurney’s point lies two-thirds laterally along a line drawn from the umbilicus to the anterior superior iliac spine. It guides the positioning for an appendicectomy incision (non-pregnant) and needle entry for a paracentesis must pass lateral to this point to avoid the inferior epigastric vessels.

  • Langer’s (cleavage or tension) lines of the skin result from the collagen fibre arrangements, and incisions placed along these heal with minimum scarring. On the anterior abdominal wall, they lie transversely.

  • The dermatomes of the anterior abdominal wall are relevant in situations of referred pain, and in the assessment of regional anaesthesia. They are illustrated in Fig. 5.3 .

The Abdominal Wall

This is essentially muscular, maintaining tone and imposing a positive intra-abdominal pressure (+5 mmHg), despite respiration.

  • A muscular cylinder joins two bony rings (costal margin and pelvis) which are joined/splinted apart by the vertebral column

  • The superior bony ring is closed off by the muscular diaphragm

  • The inferior ring is closed off by the muscular pelvic ‘diaphragm’/pelvic floor.

Clinical Application

At laparoscopy the intra-abdominal pressure should always be noted together with its fluctuation with respirations. The Veress needle and trocar should be angled inferiorly at 45 degrees from the umbilicus in the midline, thus avoiding the aorta (which has already terminated) and the iliac vessels (which have diverged).

The muscles of the abdominal wall can be thought of as straight (anterior and posterior) and flat (lateral) muscles ( Fig. 5.11 ).

Aug 6, 2023 | Posted by in OBSTETRICS | Comments Off on Applied Anatomy

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