150 THE SKXSORIAL FUxXCTloX. 



through which the continuation of the brain, termed the spinal cord or 

 marrow, passes out of the skull. 



As an additional contrivance to support the enormous Aveight of the 

 head, are two other projections of the occipital bone, pecuhar to animals 

 whose heads are set on in a slanting direction, and to which powerful 

 muscles are inserted. They are called the coracoid, beak-like, processes 

 or prolongations of the occipital bone. 



Running forward, and forming outwardly a part of the base, and in- 

 wardly a portion of the floor of the skull, is what, from, its wedge-like 

 shape, is called the basilar pi'ocess of the occipital bone. It is thick, 

 strong, and solid, and placed at the bottom of the skull, not only to be a 

 proper foundation for, and to give additional strength to the arch on either 

 side, but speedily to stop all vibration and concussion. 



At the base of the skull, and anterior to or below the occipital, hes the 

 sphenoid, wedge- like bone. This bone branches out into four irregular 

 bodies or plates, two of which are called the wings, and two running to 

 the palate, the legs. There is nothing important belonging to them, so far 

 as this work is concerned. Intei'nally the sphenoid forms a portion of the 

 cavity of the skull. 



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 pai't of it is that which is composed of 

 a great number of tliin convoluted plates, forming numerous cavities or 

 cells lined with the mucous membrane of the nose, and entering into its 

 cavity. The upper portion is called the cribriform or sieve-shape plate, 

 from its being perforated by a multitude of httle holes, through which the 

 nerve connected with smelhng passes and spreads over the nose. ■ 



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

 tentorium stretching across it, gives it the appearance of being divided 

 into two. 



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 sohd 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 tAvo plates of bone : the outer one 

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

 be easily fractui-ed ; the inner plate being elastic. By the union of these 

 two substances of different construction, the ^dbration is lessened or 

 desti'oyed, so far as safety requires. 



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

 membrane which is at once the hniug of the cranium and the covering of 

 the brain— the dura mater — presents itself. Between this membrane, 

 common to the cranium and the bi-ain, 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 

 — it is indeed the vascular membrane of the brain, being that through 

 the medium of which the arteries convey the blood to the brain — which 

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

 depression, and clothes every rrregularity and part and portion of the brain. 



