44 STRUCTURE AND ECONOMY OF THE HORSE. 



pain, the least pressure on the medulla Is productive of injury, 

 or death. The brain is largely supplied with blood by means of 

 the carotid arteries, which is returned to the heart by the jugular 

 veins. 



The spinal marrow may be considered as the continuation 

 of the brain, running from the medulla oblongata, throughout 

 the spinal canal, to the tail. It is enveloped by the same mem- 

 branes as the brain, and continues to the sacrum, where it ends 

 in several nervous cords. Its form is cylindrical, and it has been 

 found to consist of six bands, in the centre of which there is a 

 sort of canah 



The Nerves, arising from the brain and spinal cord, are forty- 

 six pair, ten of which proceed from the brain, and the remainder 

 from the cord, and are therefore called the spinal nerves. 



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

 of white filaments, each having its particular covering, 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 nostrils. 



The second pair is the Optic, which rises 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. It 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 hearing 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 

 jmr 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 phaiynx, larynx and ocsopliagus, 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 intimately connected with life itself, 

 giving to the heart and stomach its power of motion, iudejiendent 

 of the will. 



The Spinal are compound nerves, having a double function, 

 and a twofold origin, conveying both sensation and motion. 

 They arise by numerous filaments from both the upper and 

 under surface of tlie spinal cord. The filaments coalesce, and, 

 before they immerge from the dura mater, join together, previous 

 to which, the upper nerve forms a sort of knot, called a ganglion. 



