118 THE SENSORIAL FUNCTION. 



Of the ethmoid— sieve-like -bone, little can be seen outwardly. A small 

 portion is found in the back part of the orbit and in the cavity of the cranium ; 

 but the most important part of it is that which is composed of a great number 

 of thin plates, forming numerous cavities or cells (fig. I, p. Ill), lined with the 

 membrane of the nose, and entering into its cavity. The upper portion is called 

 the cribriform or sieve-shaped plate, from its being perforated by a multitude of 

 little holes, through which the nerve connected with smelling passes and spreads 

 over the nose. 



Altogether these bones form a cavity of an irregular oval shape, but the 

 tentorium penetrating into it, gives it the appearance of being divided into two 

 (d, p. 111). 



The cavity of the skull may be said to be arched all round. The builder 

 knows the strength which is connected with the form of an arch. 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 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 one hard 

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

 fractured ; the inner plate being elastic. By the union of these two substances 

 of different construction, the vibration is damped or. destroyed, so far as safety 

 requires. 



On raising any part of the skull of the horse, the dense and strong mem- 

 brane which is at once the lining of the cranium and the covering of the brain 

 — the dura mater — presents itself. It is united to the membranes below by 

 numerous little cords or prolongations of its substance, conveying blood and 

 communicating strength to the parts beneath. Between this membrane, com- 

 mon to the cranium and the brain, and the proper investing tunic of that organ, 

 is found that delicate gossamers' web, appropriately called the arachnoid — 

 the spider's membrane — and which is seen in other animals, designed either 

 to secrete the fluid which is interposed, for the purpose of obviating injurious 

 concussion, or, perhaps, to prevent the brain from readily sympathising with any 

 inflammatory action produced by injury of the skull. 



Beneath is the proper investing membrane of the brain — the pia mater — 

 which not only covers the external surface of the brain, but penetrates into 

 every depression, lines every ventricle, and clothes every irregularity and part 

 and portion of the brain. 



We now arrive at the brain itself. The brain of the horse corresponds with 

 the cavity in which it is placed (m, p. 111). 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 (re, p. 111). In the human being the cerebrum 

 is above the cerebellum, in the quadruped it is below; and yet in both they 

 retain the same relative situation. The cerebellum is nearer to the foramen 

 through which the brain passes out of the skull (ra, p. Ill), and the continuation 

 of the cerebrum passes under the cerebellum (y, p. Ill), in order 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. 



He who for the first time examines the brain of the horse will be struck with 

 its comparative diminutive size. The human being is not, generally speaking, 

 more than one-half or one-third of the size and weight of the horse ; yet the 

 brain of the biped is twice as large and as heavy as that of the quadruped. If 

 it had been the brain of the ox that had been here exposed, instead of that of 

 the horse, it would not have been of half the bulk of that of the horse. If the 



