312 STRUCTURE OF THE SHEEP. 



On examining a nerve, we find that it consists of a vast 

 number of white filaments, each having its particular cover- 

 ing, and yet compactly bound together and invested by 

 membrane. 



Of the cranial nerves the first pair is the olfactory, the 

 nerve of smelling, pulpy in its structure, and the largest in 

 the body. It rises from the cerebrum, passes out of the 

 cranium, and is spread out on the membrane lining the nos- 

 trils. 



The second pair, the optic, rise from the cerebrum, but, 

 before they pass out of the skull, join together and decussate, 

 the right nerve going to the left eye, and vice versa. Each 

 takes an oblique course, pierces the outer coats of the eye, 

 and is spread out in the form of the retina, and thus conveys 

 the impressions of objects to the brain. The sense of hear- 

 ing is supplied by a soft nerve, the auditory, which enters 

 an orifice in the temporal bone, where the seat of hearing is 

 contained. The sense of taste is supplied by the fifth, 

 which is a compound nerve, conveying both sensation and 

 motion. The other cranial nerves convey sensation and 

 motion to the various parts of the head ; but there is one 

 nerve which demands more particular notice. This is the 

 par vagum, or pneumo-gastric of the French. It rises from 

 the brain, passes down the neck close to the carotid artery, 

 and distributes branches to the pharynx, larynx, and oeso- 

 phagus, heart, lungs, stomach, and liver. If divided on 

 both sides in the living animal, death immediately ensues. 

 Its importance may thus be readily conceived ; it is inti- 

 mately connected with life itself, giving to the heart and 

 stomach their power of motion, independent of the will. 

 The spinal are compound nerves ; having a double function 

 and a twofold origin, they convey both sensation and motion. 

 They arise by numerous filaments from both the upper and 

 under surface of the spinal cord. The filaments coalesce, 

 and, before they emerge from the dura mater, join together, 

 previous to which the upper nerve forms a sort of knot called 

 a ganglion. This latter is the nerve of sensation, the other 

 the nerve of motion ; and thus, though united together, the 

 filaments are yet distinct, and a part is endowed mostly with 

 sensation, or with motion, according as the filaments of the 

 former or the latter predominate. 



There is yet another nerve which requires to be noticed 



