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Directional terms in anatomy are referenced to the median plane: medial means toward the midline, while lateral means away from it. Proximal and distal describe position along a limb relative to its attachment to the trunk, and superficial/deep describe distance from the body surface.
Skeletal muscle fibers show cross-striations from organized sarcomeres and are innervated by somatic motor neurons, giving voluntary control. Cardiac muscle is striated but involuntary and autorhythmic, while smooth muscle and myoepithelium are nonstriated.
Atrioventricular valves guard the openings between atria and ventricles: the right side has the tricuspid valve and the left side has the bicuspid, or mitral, valve. The pulmonary and aortic valves are semilunar valves located at the outflow of the right and left ventricles, respectively.
The adult vertebral column has 7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral, and 3 to 5 fused coccygeal segments. The cervical count of seven is constant in humans and is the region bearing the transverse foramina.
Sesamoid bones develop within a tendon and serve to protect the tendon and improve its mechanical advantage; the patella, embedded in the quadriceps tendon, is the largest example. The calcaneus is a short irregular tarsal bone, the sternum is a flat bone, and the sphenoid is an irregular skull bone.
Epidermal layers are named in order from deep to superficial as basale, spinosum, granulosum, and corneum. Cells formed in the basal layer migrate outward, keratinize, and are shed from the stratum corneum at the surface.
Calcitonin is produced by parafollicular (C) cells of the thyroid gland and lowers serum calcium by inhibiting osteoclastic resorption. The parathyroids secrete parathyroid hormone, which has the opposite effect on serum calcium.
The pulmonary veins are the exception to the rule that veins carry deoxygenated blood: they return oxygenated blood from the lungs to the left atrium. The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs, and the venae cavae and coronary sinus return deoxygenated blood to the right atrium.
Gas exchange requires the thinnest possible barrier, which is provided by a single layer of flattened cells resting on a basal lamina. Stratified and taller epithelia increase diffusion distance and are suited instead to protection, secretion, or transport.
The brachial plexus arises from the anterior rami of C5 through T1 and organizes into trunks, divisions, cords, and terminal branches supplying the upper limb. The cervical plexus arises from C1 through C4, and thoracic anterior rami form the intercostal nerves.
The facial nerve (VII) provides motor innervation to the muscles of facial expression derived from the second pharyngeal arch, and it also carries taste from the anterior two-thirds of the tongue. The trigeminal nerve supplies the muscles of mastication, the hypoglossal nerve supplies the tongue musculature, and the glossopharyngeal nerve supplies pharyngeal muscles.
The right lung has three lobes separated by two fissures: the oblique fissure divides the inferior lobe from the rest, and the horizontal fissure separates the superior lobe from the middle lobe. The left lung has only an oblique fissure and possesses a lingula but no middle lobe.
The celiac trunk is a short unpaired ventral aortic branch that divides into the left gastric, splenic, and common hepatic arteries, supplying the foregut derivatives. The mesenteric arteries arise independently from the abdominal aorta, as do the paired renal arteries.
Bowman's capsule encloses the capillary tuft of the glomerulus, with a visceral layer of podocytes and a parietal layer of simple squamous cells; filtrate collects in the capsular space. The loop of Henle and collecting duct are downstream tubular segments, and the juxtaglomerular apparatus is a regulatory structure at the vascular pole.
Joints are classified structurally by the tissue uniting the bones. Sutures are united by dense fibrous connective tissue and are therefore fibrous joints, specifically synarthroses that permit essentially no movement.
The anterior cruciate ligament runs from the anterior intercondylar area of the tibia to the lateral femoral condyle and tightens when the tibia is pulled forward, thus limiting anterior translation. The posterior cruciate ligament resists posterior translation, and the collateral ligaments resist varus and valgus stress.
The rotator cuff comprises four scapulohumeral muscles whose tendons blend with the shoulder capsule: supraspinatus, infraspinatus, teres minor, and subscapularis. Teres major, deltoid, and latissimus dorsi act on the humerus but are not part of this cuff.
The ectoderm gives rise to the neural plate, which folds into the neural tube and neural crest, forming the central and peripheral nervous systems as well as the epidermis. Endoderm forms the gut lining and associated glands, and mesoderm forms muscle, bone, and connective tissues.
Every preganglionic autonomic fiber, whether sympathetic or parasympathetic, releases acetylcholine onto nicotinic receptors in the autonomic ganglion. Norepinephrine is released by most postganglionic sympathetic fibers, while parasympathetic postganglionic fibers release acetylcholine.
The cisterna chyli is a dilated sac at the lower end of the thoracic duct, lying anterior to the bodies of the first and second lumbar vertebrae. It receives the right and left lumbar trunks and the intestinal trunk before lymph ascends through the aortic hiatus.
The femoral triangle is bounded laterally by the medial edge of sartorius, medially by the medial edge of adductor longus, and superiorly by the inguinal ligament. Its floor is formed by iliopsoas and pectineus, and it contains the femoral nerve, artery, and vein.
Each division of the trigeminal nerve exits through a distinct skull opening: V1 through the superior orbital fissure, V2 through the foramen rotundum, and V3 through the foramen ovale. The facial nerve leaves the skull through the internal acoustic meatus and stylomastoid foramen.
In a right-dominant circulation, which occurs in the majority of people, the right coronary artery gives off the posterior interventricular branch after forming the posterior part of the coronary sulcus. This vessel supplies the posterior one-third of the interventricular septum and adjacent ventricular walls.
The portal vein forms behind the neck of the pancreas by union of the superior mesenteric vein with the splenic vein, and it carries absorbed nutrients to the hepatic sinusoids. The inferior mesenteric vein usually drains into the splenic vein rather than joining the portal confluence directly.
The sciatic nerve arises from the sacral plexus as the union of the tibial and common fibular divisions, carrying fibers from anterior rami of L4 through S3. It is the largest nerve in the body and exits the pelvis through the greater sciatic foramen.
The adult vertebral column contains seven cervical, twelve thoracic, and five lumbar vertebrae, with the sacrum and coccyx formed by fused segments. The cervical count is fixed at seven in the typical spine.
The ligamentum flavum is an elastic ligament that spans the laminae of adjacent vertebrae and forms part of the posterior wall of the vertebral canal. Its elasticity helps the spine recoil after flexion.
The vertebral artery and vein pass through the transverse foramina of C6 through C1 before entering the foramen magnum. This course is a defining feature of cervical vertebrae.
The dens is a superior projection from the body of the axis (C2) that articulates with the anterior arch of the atlas. It serves as a pivot for head rotation.
The thoracic and sacral curves are primary kyphoses present from fetal life. The cervical and lumbar curves are secondary lordoses that develop after birth.
The adult spinal cord ends as the conus medullaris near the L1-L2 disc level. Below this level the cauda equina occupies the lumbar cistern.
Dorsal roots contain sensory (afferent) fibers whose cell bodies lie in the dorsal root ganglion. Ventral roots contain motor (efferent) fibers.
The anterior longitudinal ligament is a strong band along the anterior aspect of vertebral bodies and discs, and it limits extension. It is the counterpart of the posterior longitudinal ligament within the canal.
C7 has a long, nonbifid spinous process that is palpable at the base of the neck and is therefore called the vertebra prominens. It is a key surface landmark.
The atlantoaxial joints are oriented to allow rotation of the head, as in indicating 'no'. Flexion and extension occur mainly at the atlanto-occipital joint.
The intervertebral foramen is bounded anteriorly by the vertebral body and intervertebral disc. Posteriorly it is bounded by the zygapophyseal joint and ligamentum flavum.
The nucleus pulposus is a gel-like center rich in water and proteoglycans. The annulus fibrosus is predominantly type I collagen.
The supraspinous ligament runs along the tips of the spinous processes and resists hyperflexion. The anterior longitudinal ligament resists extension.
Lumbar facet joints are oriented sagittally, which permits flexion-extension and limits rotation. Thoracic facets are more coronal and cervical facets are oblique.
Lumbar puncture is performed below the conus medullaris, typically at L3-L4 or L4-L5, where the cauda equina floats in cerebrospinal fluid. This reduces the risk of cord injury.
The artery of Adamkiewicz is a major radiculomedullary artery from a lower intercostal or lumbar artery. It supplies the lower thoracic and lumbar spinal cord.
The denticulate ligament is a lateral extension of pia mater that suspends the spinal cord within the dural sac. It projects through the arachnoid to attach to the dura.
The adult disc is avascular, so nutrients diffuse from capillaries in the vertebral body endplates and the outer annulus. This limited diffusion helps explain disc degeneration.
A typical rib such as the first rib articulates with a vertebral body and with the transverse process of the numerically corresponding vertebra. Floating ribs lack the typical transverse process articulation.
Uncovertebral joints form between the uncinate processes of the lower cervical vertebral body and the body above. They lie at the lateral margins of the intervertebral disc.
The pars interarticularis forms the neck of the Scottie dog on oblique lumbar radiographs. A pars defect appears as a collar around the neck.
The tectorial membrane is the upward continuation of the posterior longitudinal ligament. It covers the dens and the cruciate ligament of the atlas.
The alar ligaments extend from the dens to the medial occipital condyles and limit axial rotation and lateral flexion. The transverse ligament limits anterior translation of the dens.
The epidural space contains adipose tissue and the internal vertebral venous plexus. Cerebrospinal fluid lies in the subarachnoid space deep to the dura.
The posterior longitudinal ligament is broad centrally but thin and deficient laterally. As a result, the posterolateral annulus is less reinforced and is the usual site of herniation.