108 ANATOMICAL STEUCTUEE OF THE SHEEP. 



those of the brain. In form the spinal cord is cylindrical, com- 

 posed of bundles of nerve fibres, with a fissure or canal passing 

 through its center. 



We find that in sheep forty pair of nerves originate from 

 the brain and spinal cord, ten arising from the brain, the balance 

 being from the cord, are hence called spinal nerves. 



Nerves are an aggregation of nerve tubes, the tubes being 

 corn-posed of sections of nerve cells enveloped by a membrane 

 called the sheath of Schwann. 



Of the cranial nerves the first pair are the olfactory, which 

 conduct the sense of smell to the cerebrum, arising in the cere- 

 brum they pass o\Tt of the cranial cavity through the ethmoid 

 bone, to be distributed throughout the mucous membrane lining 

 the nostrils. 



The second pair are termed the optic. They also originate 

 in the cerebrum from opposite hemispheres, joining before leav- 

 ing the skull, when the right nerve passes to the left eye, and 

 the left to the right eye by a process termed decussation, taking 

 an oblique course, they pierce the outer coats of the eye and 

 spread out over the retina, from whence impressions of objects 

 are conveyed to the brain. The sense of taste is supplied by the 

 fifth pair of nerves, that of hearing by the auditory, a portion of 

 the seventh pair; the other cranial nerves conveying motion and 

 sensation to the various parts of the head. 



Another nerve of the cranium which deserves special men- 

 tion, being the most important of them all, is the tenth pair, or 

 pneumogastric, remarkable for its extent and for the multiplicity 

 of the physiological uses ascribed to it, being distributed to the 

 gullet, throat, lungs, bronchial tubes, windpipe, and stomach. 

 It controls the motions and secretions of the organs to which it 

 is distributed, having special power over the heart's action. 

 Special nerves are termed compound nerves with a double func- 

 tion and twofold origin, conveying both sensation and motion. 



