Human Anatomy - Kotsan I. Y. 2009

Brain
General overview of the brain

The Brain (encephalon) and its protective membranes are housed within the cranial cavity, adopting a shape that generally conforms to the internal contours of the Skull. Its superolateral surface is convex to match the cranial vault, while the inferior surface, or the Base of the brain, is somewhat flattened and exhibits a complex relief that mirrors the shape of the cranial fossae at the base of the internal skull.

The mass of an adult human brain ranges from 1,100 to 2,500 g; on average, it is 1,400 g in men and 1,245 g in women. The sagittal dimension of an adult brain is 16–17 cm, the transverse dimension is 13–14 cm, and the vertical dimension ranges from 10.5 to 12.5 cm. The volume of the brain averages 1200 cm³. The brain reaches its maximum mass and volume at the age of 20, remaining relatively stable until the age of 60. After 60, brain mass and volume experience a slight decline.

Reflecting The Development of the five primary brain vesicles, the human brain is divided into 5 distinct regions: 1 — Telencephalon (cerebrum); 2 — Diencephalon; 3 — mesencephalon (Midbrain); 4 — metencephalon (Hindbrain); 5 — myelencephalon (Medulla Oblongata), which transitions into the Spinal Cord at the level of the foramen magnum (Fig. 238).

Topographically, three phylogenetically, functionally, and anatomically distinct PARTS OF THE brain are recognized: the cerebral hemispheres (or cerebrum), the Cerebellum, and the Brainstem.

The cerebral hemispheres (hemispheriae cerebrales) represent the most highly developed part of the Central Nervous system. They overlie all other structures of the brain, forming the so-called mantle (Pallium). Three surfaces are distinguished on the cerebral hemispheres: superolateral, medial, and inferior.

The right and left hemispheres are separated by the deep longitudinal cerebral fissure (fissura longitudinalis cerebralis). Deep within this fissure lies the corpus callosum—a massive neural tract (commissure) connecting the two hemispheres. Anteriorly, the longitudinal fissure is completely open. Posteriorly, it connects with the transverse cerebral fissure (fissura transversa cerebralis), which separates the cerebral hemispheres from the cerebellum.

The superolateral, medial, and inferior (basal) surfaces of the cerebral hemispheres are marked by deep and shallow sulci (sulci cerebrales). Deep sulci divide each hemisphere into cerebral lobes (lobi cerebrales), whereas shallow sulci separate the individual cerebral gyri (gyri cerebrales).

Fig. 238. Regions of the brain shown in a median sagittal section

1 — Pituitary Gland (hypophysis); 2 — tuber cinereum; 3 — optic chiasm; 4 — Third ventricle; 5 — anterior commissure; 6 — septum pellucidum; 7 — fornix; 8 — corpus callosum; 9 — interthalamic adhesion; 10 — thalamus (optic mound); 11 — posterior (epithalamic) commissure; 12 — Pineal Gland (epiphysis); 13 — tectum of midbrain; 14 — cerebral peduncle; 15 — cerebral aqueduct; 16 — Pons; 17 — Fourth ventricle; 18 — cerebellum; 19 — medulla oblongata

The Inferior surface of the brain reveals not only the basal aspects of the cerebral hemispheres and cerebellum, but also the entire inferior surface of the brainstem, as well as the emerging Cranial Nerves (Fig. 239).

The anterior region of the inferior surface of the brain is formed by the frontal lobes of the hemispheres. Positioned on the inferior surface of these lobes are the olfactory bulbs (bulbi olfactorii), which appear as small enlargements situated on either side of the longitudinal cerebral fissure. Passing from the Nasal cavity through the foramina of the cribriform plate, 15–20 delicate nerve filaments approach the ventral surface of each olfactory bulb, collectively forming the 1st pair of cranial nerves — the olfactory nerves (nn. olfactorii). When the brain is removed from the skull, these fragile filaments tear away from the bulbs and are therefore invisible on an isolated brain specimen. Extending posteriorly from each olfactory bulb is the olfactory tract (tractus olfactorius). As the posterior part of the olfactory tract widens, it forms the olfactory trigone (trigonum olfactorium). The posterior boundary of the olfactory trigone transitions into a small area perforated by numerous tiny openings, known as the anterior perforated substance (substantia perforata anterior), through which Arteries penetrate the deep structures of the brain. Medial to this perforated substance, closing the posterior extent of the longitudinal cerebral fissure on the base of the brain, lies a thin, grayish layer termed the terminal lamina (lamina terminalis). Posteriorly, this lamina is adjacent to the X-shaped optic chiasm (chiasma opticum). The optic chiasm is formed by fibers running within the optic nerves (n. opticus) — the 2nd pair of cranial nerves, which enter

the cranial cavity from the orbits. Two optic tracts (tractus opticus) diverge posteriorly and laterally from the optic chiasm.

Fig. 239. Base of the brain and sites of emergence of cranial nerve roots (after M. R. Sapin)

1 — olfactory bulb; 2 — olfactory tract; 3 — anterior perforated substance; 4 — tuber cinereum; 5 — optic tract; 6 — mammillary bodies; 7 — trigeminal ganglion; 8 — posterior perforated substance; 9 — pons; 10 — cerebellum; 11 — pyramid; 12 — olive; 13 — spinal nerve; 14 — Hypoglossal nerve; 15 — Accessory nerve; 16 — Vagus nerve; 17 — Glossopharyngeal nerve; 18 — Vestibulocochlear nerve; 19 — Facial Nerve; 20 — Abducens nerve; 21 — Trigeminal nerve; 22 — Trochlear nerve; 23 — Oculomotor nerve; 24 — Optic nerve; 25 — olfactory sulcus

Posterior to the optic chiasm lies the tuber cinereum, whose apex points downward and tapers into a narrow funnel called the infundibulum. Suspended from the infundibulum is the pituitary gland, or hypophysis (hypophysis cerebri). This endocrine gland rests within the hypophysial fossa (sella turcica) of the Sphenoid bone and typically detaches from the infundibulum, remaining in the bony fossa when the brain is extracted from the skull.

Posterior to the tuber cinereum are two paired, rounded white elevations known as the mammillary bodies (corpora mamillaria). Behind them lies a relatively deep interpeduncular fossa (fossa interpeduncularis), bounded laterally by two thick ridges converging posteriorly, called the cerebral peduncles (pedunculi cerebri). The floor of this fossa is pierced by vascular openings and is therefore designated as the posterior perforated (interpeduncular) substance (substantia perforata (interpeduncularis) posterior).

Along the medial, mutually facing surfaces of the cerebral peduncles, the roots of the right and left oculomotor nerves (n. oculomotorius) — the 3rd pair of cranial nerves — are visible. The lateral surfaces of the peduncles are curved around by the trochlear nerves (n. trochlearis) — the 4th pair of cranial nerves, whose roots emerge not from the base of the brain like the other 11 pairs, but from the dorsal surface just posterior to the inferior colliculi of the midbrain tectum, lateral to the frenulum of the superior medullary velum.

The cerebral peduncles emerge from the upper borders of a broad transverse band of nerve fibers known as the pons. The lateral portions of the pons extend into the cerebellum, forming the paired middle cerebellar peduncles (pedunculi cerebellaris medii).

At the junction between the pons and the middle cerebellar peduncles, the 5th pair of cranial nerves — the trigeminal nerve (n. trigeminus) — emerges bilaterally.

Posterior (inferior) to the pons lie the anterior regions of the medulla oblongata, characterized by medially positioned pyramids (pyramides medullae oblongatae) separated by the anterior median fissure, alongside rounded lateral elevations known as the olives (olivae).

At the bulbopontine junction (between the pons and medulla), lateral to the anterior median fissure, the roots of the abducens nerve (n. abducens) — the 6th pair of cranial nerves — exit the brain. More laterally, situated sequentially between the middle cerebellar peduncle and the olive, are the roots of the facial nerve (n. facialis) — the 7th pair, and the vestibulocochlear nerve (n. vestibulocochlearis) — the 8th pair of cranial nerves.

Dorsal to the olive, within a subtle posterolateral sulcus, the roots of the glossopharyngeal nerve (n. glossopharyngeus) — the 9th pair, the vagus nerve (n. vagus) — the 10th pair, and the accessory nerve (n. accessorius) — the 11th pair of cranial nerves are arranged from rostral to caudal. Additional rootlets of the accessory nerve also arise from the upper cervical segments of the spinal cord. In the sulcus separating the pyramid from the olive lie the rootlets of the hypoglossal nerve (n. hypoglossus) — the 12th pair.

A median sagittal section of the brain, cut along the longitudinal cerebral fissure, reveals the Medial surface of the cerebral hemisphere along with key structural details of the brainstem and cerebellum.

The medial surface of the cerebral hemispheres, much like their other surfaces, features sulci that delineate the individual cerebral gyri.

The regions of the frontal, parietal, and occipital lobes in each hemisphere are separated from the prominent corpus callosum by the corresponding sulcus of the corpus callosum (sulcus corporis callosi).

The corpus callosum, or great cerebral commissure, consists of numerous densely packed nerve fibers that connect symmetrical areas of the right and left cerebral hemispheres, forming the main part of the Commissural pathways.

The middle part of the corpus callosum is called the trunk (truncus corporis callosi), the anterior sections of which curve downward to form the genu of the corpus callosum (genu corporis callosi). Extending further downward, the genu thins out into the rostrum of the corpus callosum (rostrum corporis callosi), which continues into the lamina terminalis. The posterior sections of the corpus callosum are thickened and terminate freely in the splenium (splenium corporis callosi).

Radiating in all directions from the corpus callosum to the Cerebral Cortex of the hemispheres, the nerve fibers form the radiation of the corpus callosum (radiatio corporis callosi). Fibers from the anterior part (genu) of the corpus callosum connect the cortex of the frontal lobes of the right and left hemispheres. Fibers from the central part of the corpus callosum—the trunk—connect the Gray matter of the parietal and temporal lobes. The splenium contains fibers connecting the cortex of the occipital lobes. The part of the corpus callosum radiation that extends laterally and downward, covering the posterior and inferior horns of the lateral ventricle, is called the tapetum. The upper surface of the corpus callosum is covered by a thin layer of gray matter known as the indusium griseum.

Beneath the corpus callosum lies a thin white lamina known as the fornix (Fig. 240).

Fig. 240. Fornix of the brain and hippocampus

1 — corpus callosum; 2 — body of the fornix; 3 — crura of the fornix; 4 — anterior commissure; 5 — Column of the fornix; 6 — mamillary body; 7 — fimbria of the hippocampus; 8 — uncus; 9 — dentate gyrus; 10 — parahippocampal gyrus; 11 — crus of the hippocampus; 12 — hippocampus; 13 — lateral ventricle; 14 — calcar avis; 15 — commissure of the fornix; 16 — collateral trigone

Gradually moving away from the corpus callosum, the body of the fornix (corpus fornicis) forms an arched curve forward and downward, transitioning into the column of the fornix (columna fornicis). The lower parts of each column initially approach the lamina terminalis, then diverge laterally and, extending backward, terminate in the mamillary bodies. Between the crura of the fornix posteriorly and the lamina terminalis anteriorly lies a transversely oriented bundle of nerve fibers that appears as an oval white Structure in cross-section: the anterior commissure (commissura rostralis). Like the fibers of the corpus callosum, this commissure connects the cerebral hemispheres to each other.

The area bounded above and anteriorly by the corpus callosum, inferiorly by its rostrum, the lamina terminalis, and the anterior commissure, and posteriorly by the crus of the fornix, is occupied by a thin sheet of Nervous Tissue: the septum pellucidum (lamina septi pellucidi).

All the aforementioned structures belong to the telencephalon. The structures located below these, with the exception of the cerebellum, belong to the brainstem.

The anterior sections of the brainstem are formed by the right and left dorsal thalami. The dorsal thalamus (thalamus dorsalis) is commonly referred to as the optic thalamus. The thalami are located inferior to the body of the fornix and the corpus callosum, and posterior to the crura of the fornix. On the median sagittal section of the brain, only the medial surface of the dorsal thalamus is visible, marked by the interthalamic adhesion (adhesio interthalamica). The medial surface of each dorsal thalamus bounds the slit-like, vertically oriented cavity of the third ventricle laterally. Between the anterior end of the thalamus and the column of the fornix lies the interventricular foramen (foramen interverntriculare), which connects the lateral ventricle of the cerebral hemisphere with the cavity of the third ventricle. Extending posteriorly from the interventricular foramen and curving inferiorly around the thalamus is the hypothalamic sulcus (sulcus hypothalamicus). Structures located ventral to this sulcus belong to the Hypothalamus. These include the optic chiasm, tuber cinereum, infundibulum, pituitary gland, mamillary bodies, and other brain structures.

Superior and posterior to the thalamus, beneath the splenium of the corpus callosum, lies the pineal gland (corpus pineale). The anteroinferior parts of the pineal body fuse with a delicate tract known as the epithalamic (posterior) commissure (commissura epithalamica (posterior)). The thalamus and its adjacent structures discussed previously belong to the diencephalon.

Caudal to the thalamus are structures belonging to the midbrain. Below the pineal gland lies the plate of the quadrigeminal bodies—the roof of the midbrain (tectum mesencephali), which consists of paired superior and inferior colliculi. Ventral to the tectal plate are the cerebral peduncles (pedunculi cerebri), separated from the plate by the cerebral aqueduct. The cerebral aqueduct (aqueductus mesencephali) connects the cavity of the third ventricle with the fourth.

Further posteriorly lie the pons and cerebellum, which belong to the hindbrain, along with the medulla oblongata. The cavity of these brain regions is the fourth ventricle. The floor of the ventricle is formed by the dorsal surfaces of the pons and medulla oblongata, which together make up the Rhomboid fossa (fossa rhomboidea). A thin sheet of White matter extending from the cerebellum to the roof of the midbrain is called the superior medullary velum (vellum medullare superius). From the inferior surface of the cerebellum posteriorly toward the medulla oblongata extends the inferior medullary velum (vellum medullare inferius).