EXTERNAL STRUCTURE. 75 



If properly constructed, it is equal to a solid mass of masonry. The arc>» 

 of the horse's skull has not much weight to support, but it is exposed to 

 many injuries from the brutality of those by whom he should be protected, 

 and from accidental causes. 



The roof of the skull is composed of two plates of bone : the outer, hard 

 and tough, and the different parts dove-tailed together, so as not to be easily 

 fractured ; the inner plate is elastic, and by the union of these two sub- 

 stances of different construction, the vibration is partly damped or destroyed. 

 By means of the elasticity of the inner plate, the force or influence which 

 might reach it through the outer plate, and, notwithstanding its difference 

 of structure, affect it, is spread over the whole of the roof; and the inner 

 plate is not dove-tailed like the outer, because the dove-tailing would impede 

 the spread of the vibration. 



The brain of the horse corresponds with the cavity in which it is placed 

 (m, p. 68). It is a flattened oval. It is divided into two parts, one much 

 larger than the other, the cerehrum or brain, and the cerehellum or little 

 brain (n, p. 68). In the human being the cerebrum is above the cerebel- 

 lum, in the quadruped below ; and yet in both they retain the same relative 

 situation. The cerebellum is nearer to the foramen or hole, through which 

 the brain passes out of the skull {ii, p. 68), and to the continuation of the 

 cerebrum passes under the cerebellum (j>. p. 68) to ari-ive at this foramen. 

 In the human head this foramen is at the base of the skull, but in the 

 quadruped, in whom the head is placed slanting, it is necessarily elevated. 



It would occupy too great a portion of our time fully to consider the 

 wonderful and mysterious functions discharged by the brain, but some 

 diseases to which the horse is subject, and a very useful operation, the divi- 

 sion of the nerve of the leg, for foot lameness, could not be understood with- 

 out a slight account of this important organ. 



When the brain is cut it presents two distinct substances (m, p. 68), one 

 principally on the outside, grey, or ash-coloured, and therefore called the 

 cortical (bark-like, or rind-like) from its situation, or the cineritious (ash- 

 like), from its colour ; and one more in the centre, and its fibres running 

 towards the centre, and white and pulpy, and from its consistence called the 

 medullary (narrow-like) part. This latter portion seems to be collected and 

 condensed towards the centre or base of the brain, and all the nerves derive 

 their origin from it. 



The medullary portion then is evidently connected with the nervous 

 system ; and the nerves are concerned in the discharge of all the offices of 

 life. They give motion to the limbs ; they supply with energy the heart, 

 the lungs, the stomach, and every part connected with life: and being the 

 medium through which sensation is conveyed, they supply the mind with 

 materials to think and work upon. 



The cineritious part has a different appearance, and is evidently differently 

 constituted ; and some have supposed it to be the residence of the mind, 

 receiving the impressions which are conveyed to the brain by the nerve of 

 sensation, and directing the operation and action of those which give motion 

 to the limbs. In accordance with this it happens that where supcior intel- 

 ligence is found, the cineritious prevails, and where little beside brutal 

 strength and appetite exist, the medullary portion is enlarged. 



From the medullary substance proceed certain cords or prolongations, 

 called nerves, by which the animal is enabled to receive impressions from 

 surrounding objects, and to connect himself with them, and to possess many 

 pleasurable or painful sensations. One is spread over the membrane of the 

 nose, and gives the sense of smell j another expands on the back of the eye, 



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