EXTERNAL STRUCTURE. 75 



If properly constructed, it is equal to a solid mass of masonry. The arch 

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

 many injuries from the brutahty 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 frac- 

 tured ; the inner plate is elastic, and by the union of these two substances 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 struc- 

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

 is not dove-tailed Hke 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 

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

 larger than the other, the cerebrum or brain, and the cerebellum or little 

 brain (ii, 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 (/?, p. 68), and the continuation of the 

 cerebrum passes under the cerebellum {p, p. 68) to arrive 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 di- 

 vision of the nerve of the leg, for foot lameness, could not be understood 

 without 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-Hke), 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 (marrow-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 differ- 

 ently 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 

 superior intelligence is found, the cineritious prevails, and where Httle 

 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 ; another expands on the back of the 



