The puns Varolii, or bridge, lies just above the medulla. Superficially its ventral surface is seen to be composed of fibres which pass trans versely, forming a sort of bridge between the two cerebellar hemispheres. Internally the pons is composed of tracts of fibres (white matter) and of nuclei (masses of gray matter). The fibre tracts pass both longitudinally and transversely. The longitudinal fibres are mainly the tion upward of tracts already mentioned in the description of the medulla, the pyramidal tract or main motor tract. and the fillet or main sensory tract. Sonic of the longitudinal fibres are, however. fibres connecting the nuclei pontis wit h higher centres a ndcommissural fibres between the nuclei pontis. The transverse fibres of the pons are mostly fibre- connecting the cerebellum with the nuclei pontis.and are consequently often called the middle cerebellar peduncles. They are composed of two sets of fibres. from their location are known respectively as the superficial transverse pontile fibres and the deep transverse pontile fibres. in the median line the fibres decussate. forming a median raptly eon t inuons with that of the medulla. Fibres of the roots of the fifth and some of the higher fibres of the roots of the sixth and seventh nerves pass front their nuclei of origin to the ventro-lateral surface of the pions. The gray matter of the pons consists of the nuclei of the nerves just mentioned and of nuclei scattered throughout the pons, which are known as the nuclei /midis and are connected with the middle cerebellar pe duncle.: and with the cerebrum. Year the pos terior surface of the polls is the fourth ventricle, which becomes narrow in the upper part of the pn• as it approaches the aqueduct of t-tylcins in the door of the ventrbde is a group of nerve Bells which are so deeply pigmented as to present a dark appearance to the naked eye. This dark area is called the locus cal-Wens.
Above the pins is the mid brain. Ilere the long fibre tracts are collected into two main bundles known as the cerebral pedimeles or ernra eerebri. Each peduncle, or erns, consists of a ventral por tion or crusta, a dorsal portion or tegmentum, and, between these, a mass of gray matter, the substantia nigra. The dorsal part of the cerebral peduncle, or tegmentum, represents the continua tion brainward of the main sensory tract to the cortex cerebri. Of the ventral part of the pe duncles, or crusta, about the middle three-fifths are taken up by the fibres of the pyramidal sys tem (including fibres to the motor nuclei of the cranial nerves). .Medial to these fibres in the peduncle are the fibres which pass from the frontal lobe to t he attelei 'totals, while external to the pyramidal fibres are the fibres connecting the temporal lobe with the nuclei pontis. As the peduneles approach the basal ganglia the sub stautia nigra disappears and the tegmentum lies just dorsal to the crusta. These bundles of fibres pass through the basal ganglia between the nucleus caudatus and the optic thalamus on the mesial side and the nucleus lenticularis on the lateral side. here they form the internal cap sule, which is directly continuous above with the corona radiata, through which the fibres enter the cortex eerebri. A bend divides the capsule into an anterior portion and a posterior portion. The anterior portion lies between the caudate nucleus internally and the lenticular nucleus externally. This part of the capsule consists mainly of fibres which connect the cortex ecrebri and the optic thalamus. The posterior portion of the internal capsule lies between the lenticular nucleus on its outer side and the optic thalamus on its inner side. About the anterior two-thirds of this portion is occupied by the fibres of the pyramidal tracts (including descending fibres to the motor cranial nerve nuclei). The cortieo pontal fibres connecting the cortex eerebri with the nuclei poutis pass through the internal cap snle in two separate bundles; one bundle, coming from the frontal lobe, passes down in front of the main motor tract; the other bundle, coining from the temporal lobe, passes through the pos terior portion of the internal capsule just behind the pyramidal tract. Through the posterior part of the internal capsule also passes the continua tion upward of the tegmentum or main sensory tract to the cortex. On the posterior surface of the mid-brain are the corpora quadrigemina or optic lobes. These are four uu number, two anterior and two posterior. Their structure is complex. The anterior pair seems to be connected in some way with vision. in the mid-brain the fourth ventricle becomes narrowed and is known as the iter o• aqueduct of Sylvius.
The ecrehellum—or little brain—is that part of the brain which lies in the inferior occipital fossa. It is situated beneath the occipital lobes of the cerebrum, from which it is separated by the tentorium evrebelli. The average weight of the cerebellum is about ounces. Its size rela tive to that of the cerebrum is about 1 to S. In the infant the eerebellum is relatively much smaller than in the adult. its weight being about
Js that of the cerebrum. The cerebellum meas ures from ftlf, to 4 inches in transverse diameter and about 2 inches from before backward at its centre. It consists of two hemispheres o• lateral lobes and a smaller central lobe called the vermi form process. The evrebellum is connected with other parts of the nervous system by means of bundles of fibres known as peduncles. Two of these, one on either side, the superior peduncles. connect it with the cerebrum. Two others, the in ferior peduncles or restiform hodiw,•vouncct the cerebellum with the spinal cord. The third set of peduncles. the middle peduncles, connect it with the pals. The hemispheres of the cere bellum are separated in front by a fossa which encircles the corpora quadrigemina. Posteriorly the furrow is deeper and narrower. on the superior surface of the cerebellum, between the anterior and posterior furrows, the central lobe has a very slight elevation and is called the superior vermiform process. On the inferior surface of the cerebellum the central lobe, called the inferior vermiform process, lies at the bot tom of the furrow, where it forms a distinct pro jection and is subdivided into an anterior portion. the nodule, a middle portion, the uvula, and :s posterior portion, the pyramid. Two small lobes stand out distinctly from the main bulk of the hemisphere, one, the flocculns, lying to the outer side of the nodule, the other, the tonsil. lying to the outer side of the uvula. Each hemisphere is divided transversely by a deep fissure called the great horizontal fissure, which separates it into an upper or tentoria I surface and a lower or occipital surface. In addition to the tloeculus and tonsil, the surface of the cerebellum is divided into several less distinct lobes, On the upper surface are the anterior or square lobe and the posterior or semi-lunar lobe. On the under surface may be seen from before backward the digastric lobe, the slender lobe, and the inferior posterior lobe. Besides the deeper furrows sepa rating the lobes, the entire surface of the cere bellum is crossed by a multitude of transverse which divide it into plates or lantelhe and give it a foliated or laminated appearance. On cutting into the cerebellum it is seen to con sist of a central portion or core of white matter tyhich 11•XtVIlds outward into the cortex us a of transversely disposed branching plates. These, c(mered over by a layer of gray matter, form the lamina already referred to, which on cross section present the leaf-like ap pearance known as the "arbor vitm." The white matter of the eerebellum is composed of medul lated nerve fibres whieh pass out a manner into the gray matter of the cortex. These fibres are axones coning from cells situated in the cerebellum and passing out to terminate either in some other part of the eerebellum or in some other part of the nervous system, or coining from cells located in other parts of the nervous system to their terminations in the cerebellum. The gray matter of the cerebellum is emuposed of three layers, an external or molecular layer, an internal or granular layer. and. between the two, a layer composed of a single layer of large cells, the layer of Purkinje cells. The molecular layer contains multipolar eells of various sizes known as 'basket cells' from the fact that while their dendrites pass mostly toward the surface, their aX011CS pass downward and end in a basket-like network around the Purkinje cells. The granular or nuclear layer derives its name from the fact that after the ordinary methods of staining it appears to be composed of a mass of small close ly packed cells. each cell consisting of a nucleus surrounded by an extremely small amount of protoplasm. Intermingled with these cells are inedullated and nommedullated fibres. lly the application of the method of (olgi these eells have been divided into small granule cells (by far the more numerous) and large granule cells. lu the ease of the small granule cell the den drites ramify in granular layer, while the axone terminateA freely in the molecular layer. The large granule cells, on the other hand, send their dendrites into the molecular layer, while their form n rich plexus within the granular layer. The dense network of fibres found in the granular layer is derived front the cells just described, from and col laterals of Tiirkinje tells, and from fibre, (Timing from the white matter. The cells of Porkinje form a single layer along the boundary between the molecular layer and the granular layer. From the outwardly directed part of the cell a thick neck is given oil', train which arise two main dendritic processes, which subdivide and give rise to an enormous numher of processes which ramify and terminate in the molecular layer. From the opposite pole of the cell is given off the axone which passes through the granular layer and becomes a medullated fibre of the white matter.