Page:The American Cyclopædia (1879) Volume III.djvu/196

 190 BRAIN of the will. The cerebellum, one eighth of the size of the cerebrum, is placed under the pos- terior part of the latter, from which it is sepa- rated by the tentorium ; it is composed of white and gray matter, the former occupying the interior; its convolutions have the form of parallel layers. Its central part or lobe is the only one found in fishes and reptiles ; its lateral lobes, found only in the higher animals and in man, indicate an advance in develop- ment. On a vertical section we find the white substance resembling the trunk of a tree from which branches are given off, hence called arbor vita, or tree of life. This organ is con- nected with the rest of the brain by three sets of fibres, the superior extending to the tuber- cula quadrigernina, the middle or the restiform fibres passing downward to the medulla, and the inferior or transverse (pons Varolii) pass- ing to the opposite side and forming a consider- able part of the tuber annulare ; the central lobe has aggregates of lobules on its superior surface, Fio. 4. Profile view of Human Brain, in vertical section, showing the medulla oblongata, the tuber nnnulare, the middle portion of the cerebellum with the arbor vitffi. the central parts of the cerebrum, and the convolutions on the inner surface of the hemispheres. containing both white and gray matter, the "superior vermiform processes," and on the lower surface the " inferior vermiform proces- ses." The transverse diameter of the cere- bellum is 3J to 4 inches, the length 2 to 2-J inches, and its thickness varying from 2 inches in front to less than J inch behind. For de- tails on the structure and on the intricate divi- sions of the cerebellum, the reader is referred to special works mentioned at the end of this ar- ticle. Disease of the cerebellum, when deep- seated, is generally manifested on the oppo- site side of the body ; this organ presides principally over the regulation of the voluntary movements. The restiform bodies of the me- dulla in their ascent to the hemispheres of the cerebellum diverge, leaving a lozenge- shaped cavity, the fourth ventricle, bounded above by the median cerebellar lobe, below by the olivary columns, behind by the nodule of the inferior vermiform process, in front by a portion of the superior vermiform pro- cess, called the " valve of Vieussens ; " on the floor are the white barb-like fibres of the seventh pair of nerves, passing at right angles, and called the calamus scriptorius; it is im- properly called the ventricle of the cerebel- lum, as it belongs to the medulla and is propor- tionate to it in size. The mesocephalon, or tuber annulare, embraces those portions of the brain which unite the cerebrum above, the cerebellum behind, and the medulla below ; the lower sur- face, or the pons Varolii, consists of curved transverse fibres, passing from one crus cere- belli to the other, crossing apparently over the anterior pyramids like a bridge ; they are al- ways developed in proportion to the cerebellar hemispheres, and are absent in animals having only the median lobe ; they constitute the great transverse commissure of the cerebellum, as the corpus callosum (mentioned hereafter) consti- tutes the great transverse commissure of the cerebrum ; these fibres extend more than one half of the depth of the tuber annulare. The tuber annulare, which exists in animals whose cerebellum has no hemispheres, projects from the medulla proper, and contains a nucleus of gray matter ; Longet is of opinion that this ganglion is an independent centre of sensation and motor power, and Dr. Todd states that the convulsions excited by a current of electro-mag- netism through it are not tetanic, but epileptic, or alternating with relaxations of the muscles. Situated above the tuber annulare are the quadrigeminal bodies, the two anterior being called nates, and the two posterior testes ; they are gangliform bodies, containing gray and white matter, the anterior being the larger ; these are the analogues of the optic lobes of birds, reptiles, and fishes, in which classes there is only a single pair, but of much larger size. The crura cere- belli, which apparently emerge from the pos- terior angles of the tuber annulare, derive their fibres from strands going to the testes, from those of the restiform body, and from those of the pons Varolii ; from the anterior angles of the tuber annulare diverge two similar processes of con- siderable thickness, the crura cerebri, which enter the cerebral hemispheres, and upon which each of these masses has been said by Dr. Todd to rest as a "mushroom upon its stalk." The fourth and fifth pairs of nerves are intimately connected with the tuber annulare. On mak- ing a section of the crura cerebri, three planes of nervous matter may be seen : the lower one, of fibrous matter, continuous with the tuber annulare and the anterior pyramids, passes up into the corpora striata, or striated bodies; above this is a dark mass, the locus niger, con- taining large caudate vesicles abounding in pig- ment, with nerve fibres among them ; the upper layer, of grayish matter, continuous with the central part of the medulla oblongata, or oli- vary columns, passes up into the optic thnlami. The striated bodies and optic thalami are best seen by laying open the lateral ventricles, in which they are placed, closely united to each other, the former being a little in front and outside of the latter. The striated bodies are