172 



ANATOMY AND PHYSIOLOGY OP 



which are symmetrical, both internally and 

 externally, arc denominated hemispheres. 



Cerebellum. — The cerebellum is at once 

 distinguished from the cerebrum by its 

 being only one-sixth the size of the latter. 

 Its figure is irregular : it has two oval ends, 

 and its lateral dimensions exceed its longi- 

 tudinal. It is divided into three oblong 

 lobes — a middle and two lateral. 



Medulla oblongata^ the smallest division 

 of the cerebral mass. It rests on the cunei- 

 form process of the occipital bone, and is 

 continued upward and backward to the 

 foramen magnum. 



The upper surface of the medulla oblong- 

 ata forms, with the tuber annulare, the 

 floor of the fom-th ventricle. 



Pltuatary gland, a red body, of an egg 

 form, seated upon the sella tursica, within 

 a fold of the dura mater. It has a mem- 

 branous capsule, surrounded by cellular ad- 

 hesions, by which it is firmly retained in its 

 place. 



Medulla spinalis. — The spinal marrow 

 is that extended portion of brain-like sub- 

 stance which is continued from the poste- 

 rior part of the medulla oblongata through 

 the entire length of the spinal canal. It is 

 inclosed in the same membranes that en- 

 velop the brain ; but, in addition to them, 

 the superior ligament of the spine serves as 

 a covering and defence to it below. To 

 this, and to the periosteum lining the 

 canal, its proper theca is loosely attached 

 by cellular, adipose, and gelatinous matter. 

 Its dura mater is derived from that which 

 covers the brain : in beinsr continued through 

 the foramen magnum, the membrane is 

 contracted into a cylindrical sheath which 

 loosely encases the marrow, and is generally 

 described under the denomination of theca 

 vertebralis. The arachnoid membrane and 

 pia mater have the same relation to the 

 marrow that the same membranes have to 

 the brain, of which they may be considered 

 prolongations. 



ORIGIN AND DISTRIBUTION OF THE NERVES. 



The nerves, being symmetrical in number 

 and distribution on either side of the body. 



take their origin in pairs, and these pairs 

 are numbered, and so distinguished from 

 one another, according to the order in 

 which they arise. There are forty-six pairs 

 of nerves ; ten, coming from the brain, are 

 distinguished as the cerebral nerves ; thirty- 

 sLx, from the spinal marrow, denominated 

 the spinal nerves. 



CEREBRAL NERVES. 



First pair, or olfactory nerves, arise from 

 the corpora strata, along the posterior bor- 

 ders of which bodies the medullary bands 

 or roots of them may be traced as high iip 

 as the middle lobes of the cerebrum. These 

 are the largest of the cerebral nerves, are 

 bulbous at their origin, pulpy in texture, and 

 exhibit, when cut into, comparatively to 

 their size, large cavities, which are w^alled 

 in by a layer of medullary matter, enclosed 

 wdthin a thinner one of cortical substance. 



Second pair, or optic nerves, arise from 

 the thalamia nervorum. They leave the 

 cranium through the optic foramen, and 

 pass to enter the globe of the eye, within 

 the interior of which it expands, and forms 

 the retina. In its whole course, it is en- 

 closed within a sheath prolonged from the 

 dura mater. 



Third pair, or motores oculorum, take 

 their origin by several filaments, from the 

 inward parts of the crura cerebri. The 

 trunk of the nerve first runs obliquely out- 

 ward, across the back of the crus, then 

 turns downward and enters the cavernous 

 sinus, on through the foramen lacerum-or- 

 bitale. In entering the cavity, the nerve 

 divides into two branches. The smaller 

 is generally received by the levater oculi. 

 The larger branch subdivides into several 

 others ; the longest of these runs round the 

 eyeball, and penetrates the oblique muscle. 

 Two or three others run to the abductor 

 and depressor muscles. 



Fourth pair of pathetic. — These take a 

 filamentous origin, and pass the border of 

 the tentorium, entering the cavernous sinus, 

 from thence to the orbit. Its destination 

 is the superior oblique muscle of the eye. 



Fifth pair, or par trigemini. — These are 



