The human brain weighs about 1300 gm and consists of three zones called forebrain, mid brain and the hind brain, each one controlling different organs and having different functions. The brain and spinal cord have three membranes which are called meninges. The fore brain also called cerebrum is the seat of intellect, thought, memory, emotion and will. It consist of two cerebral hemispheres which have numerous folds with a central length –wise cut. The outer layer is called the cerebral cortex. It is the seat of higher nervous activity. The right hemispheres control the impulses from the left side of the body and the left hemispheres controls the impulses from the right side of the body.The mid brain consists of thick bundles of fibres which penetrate deep into the right and left hemispheres of the brain, as well as four eminences. These nerve bundles are also called as cerebral peduncles. The mid brain controls the tone of the muscle.
The hind brain consists of the upward continuation of the spinal cord which is known as medulla oblongata. A large cylinder-like elevation called pons, and two hemispheres one on each side collectively known as cerebellum. The hind brain is very important portion of the human brain as it is here that the control centres of respiratory, circulatory, the centres of muscles that produce facial expressions, centres of swallowing, vomiting and mastication are located.
The brain stem consist of the midbrain, the pons and the medulla. In human brain, with the cerebral hemispheres the midbrain is continuous from above. The medulla is continuous, below with the spinal cord. In posterior position the pons and medulla are separated from cerebellum by the fourth ventricle. The ventricle is continuous , below with the central canal, which traverses the lower part of the medulla, and becomes continuous with the central canal of the spinal cord. in the cranial position the fourth ventricle is continuous with the aqueduct, which passes through the midbrain. The midbrain, pons and medulla are connected to the cerebellum by the superior, middle and inferior cerebellar peduncles, respectively. Cranial nerves are attached to the brainstem. The third and fourth nerves emerge from the surface of the midbrain and the fifth from the pons; the sixth, seventh and eighth nerves emerge at the junction of the pons and medulla. The ninth, tenth, eleventh and twelfth cranial nerves emerge from the surface of the medulla.
In the human brain, the medulla is broad above, where it joins the pons and narrow down below, where it becomes continuous with the spinal cord. The medulla is divided into a lower closed part, which surrounds the central canal and an open part, which is related to
the lower part of the fourth ventricle. The surface of the medulla is marked by a series of fissures or sulci that divide it into a number of regions. The upward continuations of the corresponding features seen on the spinal cord are the anterior median fissure and posterior median sulcus. On each side of the anterolateral sulcus lies in the line with ventral roots of spinal nerves. The region between the anterior median sulcus and the anterolateral sulcus is occupied by the elevation called the pyramid. In the upper part of the medulla, the region between the anterolateral and posterolateral sulci shows a prominent elongated, oval swelling named the olive. It is produced by large mass of grey matter called the inferior olivary nucleus. The posterior part of the medulla, between the posterior median sulcus, contains tracts that enter the medulla from the posterior funicular of the spinal cord. these are the fasciculus gracilis lying medially, next the middle line, and the fasciculus cuneatus lying laterally. These fasciculi end in rounded elevations called the gracile and cuneate tubercles . these tubercles are produce by masses of grey matter called the nucleus gracilis and the nucleus cuneatus respectively.
Just above these tubercles of the human brain the posterior aspect of the medulla is occupied by a triangular fossa which forms the lower part of the floor of the 4th ventricle. This fossa are bounded on either side by the inferior cerebellar peduncle. The lower part of the medulla, immediately lateral to the fasciculus cuneatus, is marked by another longitudinal elevation called the tuberculum cinereum. The grey matter of these nucleus is covered by a layer of nerve fibres.
The pons of the human brain shows a convex anterior surface, marked by prominent transversely running fibres. Laterally, these fibres collect to from the bundle, called the middle cerebellar peduncle . The trigeminal nerve emerges from the anterior surface, and the point of its emergence is taken as a landmark to define the plane of junction between the pons an middle cerebellar peduncle, In the midline, the anterior surface of the pons is marked by a shallow groove which lodges the basilar artery, namely sulcus basilaris. The line of junction between the pons and the medulla is marked by a groove through which a number of cranial nerve emerge. Just above the pyramid the abducent nerve emerges and runs upwards in close relation to the anterior surface of the pons. The posterior aspect of the pons forms the upper part of the floor of the 4th ventricle.
From the anterior aspect of human brain , if the midbrain is viewed it will be seen that two large bundles of fibres, one of each side of the middle line. These are the crura of the midbrain. By a deep fissure the crura are separated. The fissure is narrow near the pons, but broadens as the crura diverge to enter the corresponding cerebral hemispheres. Just below the cerebrum from the posterior boundary of a space called the interpeduncular fossa. The oculomotor nerve emerges from the medial aspect of the crus of the same side. The posterioraspect of the midbrain is markedby four rounded swellings; these are the colliculi, one superior and one inferior on each side. Just below the colliculi , there is the uppermost part of a membrane, the superior medullary velum, which stretches between the two superior cerebellar peduncles, and helps to form the roof of the 4th ventricle.
The cerebellum of the human brain lies in the posterior cranial fossa, behind the pons and the medulla. A fold of dura matter is separated from the cerebrum called the tentorium cerebell. The cerebellum consist of a part lying near the middle line vermis, and of two lateral hemispheres. Its two surfaces are the superior and the inferior. There is no line of distinction between vermis and hemispheres on the superior aspect. On the inferior aspect, the two hemispheres are separated by a deep depression named the vallecula. The vermis lies in the depth of this depression. The hemisphere extended beyond the vermis anteriorly and posteriorly and are separated by anterior and posterior cerebellar notches. The surface of the cerebellum is marked by a series of fissures which run more or less parallel to one another. The fissures subdivided the surface of the cerebellum into narrow bands called the folia. The long axis of the majority of folia is more or less transverse.
Some of the fissures on surface of the cerebellum are deeper than others. They divide the cerebellum into lobes. The deepest fissures in the cerebellum are the primary fissure on the superior surface, and the posterolateral fissure on the inferior aspect. The cerebellum divided into three lobes by these fissures. Those are the anterior lobe, the middle lobe and the flocculonodular lobe . The corpus cerebelli is formed together by the anterior and middle lobes.
The central core of each cerebellar hemisphere of the human brain is formed by white matter. The peduncles are continued into this white matter. By a thin lamina of fibres the white matter of the two sides are connected which are closely related to the fourth ventricle. the upper part of this lamina forms the inferior medullary velum, both of which take part in forming the roof of the fourth ventricle.
Most of the grey matter of the cerebellum of the human brain is arranged as a thin layer covering the central core of the white matter and the layer is called by cerebellar cortex. The subdivision of the cerebellar cortex correspond to the subdivisions of the cerebellum described above. Embedded within the central core of white matter which constitute the cerebellar nuclei.
The cerebrum of human brain consists of two cerebral hemisphere that are partially connected with each other. When cerebrum is separated from the hindbrain by cutting across the midbrain, and is viewed from below, it may be seen that the posterior to the midbrain the undersurface of the splenium of the corpus callosum. Anterior to the midbrain there is a depressed area called the interpeduncular fossa. The fossa is bounded in front by the optic chiasmaand on the sides by the right and left optic tracts. Anterior and medial of the crura of the midbrain there are two rounded swelling called the mamillary bodies. Anterior of these bodies there is a median elevation called the tuber cinereum . Close to the medial bordar of the orbital surface there is a anteroposterior sulcus which is called olfactory sulcus because the olfactory bulb and tract lie superficial to it.
Some area of the cerebral cortex can be assigned specific functions. The motor area of classical description is located in the pre-central gyrus on the superolateral surface of the hemisphere, and the anterior part of the paracentral lobule on the medial surface. The premotor area is located just anterior to the motor area. It occupies posterior parts of the superior, middle and inferior frontal gyre. The sensory area of classical description is located in the postcentral gyrus.The area of cortex that receives sensations from a particular part of the body is not proportional to the size of that part, but ratherto the complexity of sensations received from it. Thus the digits, the lipsand the tongue have a disproportionately large representation. The visual area of the human brain concerned with vision are located in the occipital lobe, mainly on the medial surface, both above and below the calcarine sulcus. The acoustic area or the area of hearing , is situated in the temporal lobe. It lies in that part of the superior temporal gyrus which forms the inferior wall of the posterior ramus of the lateral sulcus. the acoustic area lies in the anterior transverse temporal gyrus and extends to a small extent into the surface of hemisphere in the superior temporal gyrus.