THE MEDULLA OBLONGATA 493 



contract on receipt of the proper stimulus from the centre. Tlie spinal 

 nerves are from forty-two to forty-three in number on each .side ; namely, 

 eight pairs of cervical, seventeen pairs of dorsal, six pairs of lumbar, fivo 

 pairs of sacral nerves, and six or seven coccygeal. Each of these nerves 

 divides at once into a superior and inferior branch, the latter giving ofF a 

 small nervelet to communicate with the sympathetic, and then going on 

 to supply the lower parts of the body and the extremities. The cord varies 

 somewhat in size in the several regions of the spine. It commences at the 

 occiput of full size, then diminishes to the fifth cervical vertebra, whore 

 there is a slight swelling, after which it is gradually reduced in dimensions 

 to the loins, where it spreads out into a wide but thin plate, after which 

 it divides into its terminal bi^anches, which have been compared to a 

 horse's tail. 



THE MEDULLA OBLONGATA 



The medulla oblongata is the anterior enlarged portion of the spinal 

 cord, of a conical shape, which extends to the pons varolii. On its inferior 

 face it presents two pyramidal bodies, and on its superior two flattened 

 cords, the corpora restiformia, while between the two are the corpora 

 olivaria. On making a section of this part, the corpus olivare is seen to be 

 chiefly composed of grey matter, and is a ganglion, superadded for the 

 special purpose of establishing the respiratory function. From its lower 

 border proceed the filaments, which unite to form the hypoglossal nerve, 

 while from the upper side emerge the glosso-pharyngeal and pneumogastric 

 nerves. 



THE ENCEPHALON {h, in, «./.aX^, head) 



The cerebellum together with the cerebrum form the mass of the 

 encephalon, and they may be examined together with advantage. The 

 two completely fill the cavity of the cranium, and ai'e invested by three 

 membranes ; — the dui-a mater, fibrous and strong ; the pia mater, vascular 

 and tender ; and the arachnoid, a serous membrane of the ordinary charactei'. 

 The dura mater also dips down between the lobes of the cerebrum to form 

 a protection against lateral displacement called the falx, and is spread across 

 from one peti-ous bone to the other, constituting the tentorium cerebelli. 

 The mass of the encephalon in the horse is small as compared with that of 

 man, weighing not quite a pound and a half, while the human brain 

 averages three pounds in the male, and four or five ounces less in the 

 female. Taking into consideration that the body of the horse weighs at 

 least eight times as much as a man's, it follows that the brain of the latter 

 is relatively sixteen times as large as that of the horse. The cerebellum 

 occupies the postero-superior part as the head is usually carried, and is 

 much smaller than the cerebrum, being only one-sixth of its volume 

 Examining it from above it presents three lobes ; a middle and two latera 

 lobes. The former is prominent, and subdivided into lobuli by severa 

 grooves, constituting the anterior and posterior vermiform processes. The 



