150 OUTLINES OF EQUINE ANATOMY. 



attached along its margin. The lens itself is biconvex, 

 being most convex posteriorly. It is composed of numerous 

 concentric transparent layers, which increase in density 

 from without inwards. In the centre is a circular nodule, 

 consisting of three triangular portions. The lamince con- 

 sist of fibres denticulated at their margins and decreasing 

 in size towards each extremity. By apposition of their 

 denticulse they form firm continuous layers, of which the 

 external is liquid, and serves to allow movement of the 

 lens in its capsule ; it is termed the liquor Morgagni, but 

 its liquidity is supposed to be the result of post-mortem 

 change. 



The ciliary arteries and arteria centralis retince supply 

 the eyeball with blood ; the ciliary nerves endow it with 

 ordinary sensation ; the (yptic nerve with the special sense 

 of sight. 



The first trace of the embryo mammal is a longitudinal 

 layer of cells, and from this both above and below an 

 arch is formed. The inferior or hcemal arch is mainly 

 composed of soft structures which are supported by a 

 bony framework ; it forms the face, thorax and abdomen. 

 The superior or neural arch subsequently becomes a bony 

 canal formed of many segments connected together by 

 various structures, which posteriorly comes to a point, 

 anteriorly four of its segments, or vertebrae form a large 

 ovoid cavity with continuous walls, the CRANIUM, Twelve- 

 bones combine to form the cranium, four pairs and four 

 single bones : — 



Frontal, parietal, squamous and petrous temporal lx)nes. 

 (Pairs.) 



Occipital, ethmoid, sphenoid, and os triquatrum. 

 (Single.) 



These bones are for the most part formed of tY*-o dense 

 layers between which is cancellated structure. The central 

 portion contains large veins, and is termed the diploe, 

 the compact laminae are the vitreous tables. The imsterior 

 'j)art of the cranium is formed by the occiput through the 

 centre of which the foramen magnum passes. Inferiorly 

 on either side of this is a foramen condyloideum. The 

 roof of the cavity is formed posteriorly by the occiput 

 which is united anteriorly to os triquatrum (centrally) 

 and to the parietal bones on either side. The latter also 

 help to form the lateral ivalls of the cavity. Anteriorly 



