Page:The New International Encyclopædia 1st ed. v. 14.djvu/427

* NERVOUS SYSTEM AND BRAIN. 377 NERVOUS SYSTEM AND BRAIN. I tion or criista, a dorsal jiurtiou or ti'gmentum, ami, Ix'twcen thesc, a mass ul' {;ray matter, the substantia nigra. The dorsal part of the cerebral pcduiicle, or tegmentum, represents the continua- tion brainward of the main sensory tract to the cortex cerebri. Of the ventral jiart of the pe- duncles, or crusta, about the middle three-fifths arc 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 l)c(hincle are the fibres which pass from the frontal lol)e to the nuclei pontis, while external to tile pyramidal fibres are the fibres connecting the temporal lobe with the nuclei pontis. As the peduncles appmach the basal ganglia the sub- stantia 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 cerebri. A bend divides the capsule into an anterior portion and a posterior portion. The anterior ])ortion lies between the caudate nucleus internally and the lenticular nucleus externally. This part of the capsule consists nuiinly of fibres which connect the cortex cerebri 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 cortico- pontal fibres connecting the cortex cerebri with the nuclei pontis pass through the internal cap- sule in two separate bundles : one bundle, coming from the frontal lobe, passes down in front of the main motor tract; the other bundle, coming 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 tegmentinn or main sensory tract to the cortex. On the posterior surface of the mid-brain are the corpora quadrigemina or optic lolies. These are four in numlier. 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 or aqueduct of Sylvius. The cerebellum — or little brain — is that part of the brain which lies in the inferior occipital fossa. It is situated beneath the oeci])ital lobes of the cerebrum, from which it is separated by the tentorium ccrebelli. The average weight of the cerebellum is about 51{. ounces. Its size rela- tive to that of the cerebrum is about 1 to 8. In the infant the cerebellum is relatively much smaller than in the adult, its weight being about ^ that of the cerebrum. The cerebellum meas- ires from 31,^ to 4 inches in transverse diameter and about 2 inches from before backward at its centre. It consists of two heniisplieres or lateral lobes and a smaller central lobe called the vermi- form process. The cerebellum is connected with other parts of the nervous system by means of bimdles of fibres known as peduncli's. Two of these, one on either side, the superior peduncles, connect it with the cerebrum. Two others, the in- ferior peduncles or rcstiform bodies, connect the cerebellum with the spinal cord. The third set of peduncles, the middle peduncles, connect it with the pons. The heniisplieres of the cere- bellum are separated in front by a fossa which encircles the corpora quadrigemina. Posteriorly Ar/, :i/ Corp.cUint VERTICAL SECTION OF THE CEBEBELLU.M. Arb. vit., arbor vit«»; Oliv. hdy., olivary Lotlv ; Corp. dent., corpus dentatuni. 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 a posterior portion, the pyramid. Two small lobes stand out distinctly from the main bulk of the hemisphere, one. the flocculus, 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 SCHEME or THK CEBEBELLAR CORTEX. A, Gray or molecular lajor; /f, jA-ranulc layer; C medulla or white matter; a, enuill praunle cell; It, large >;raitule cell; c, plexus of uerve flbrcs; (/, horizontal buiidle; e, fibres of molecular layer; f, cell of I'urkinje ; ^, basket cell; b, small cortical cell; ;', glia cell of the molecular layer; i, mossy cell resembling a glia cell; /, spider cell. into an upper or tentorial surface and a lower or occipital surface. In addition to the flocculus 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 semilunar lobe. On the under surface may be seen from before backward the digastric lobe, the slender lolie, and the inferior posterior lobe. Besides the deeper furrows sepa-