870 THE SENSE ORGANS AND SKIN OF THE HORSE 



and of the posterior surface, 9.5 to 10 mm. But the curvatures of its surfaces — especially that of 

 the anterior — vary dui-ing hfe according as the eye is accommodated for near or far vision. 



The zonula ciliaris or suspensory ligament of the lens (Fig. 689) consists of 

 delicate fibers (Fibrse zonulares) which pass in a meridional direction from the 

 ciliary processes to the capsule of the equator of the lens. Many fibers cross each 

 other, and the spaces between the fibers (Spatia zonularia) are filled with aqueous 

 humor; they communicate with each other and with the posterior chamber. 



The substance of the lens (Substantia lentis) is enclosed by a structureless, 

 highly elastic membrane, the capsule of the lens (Capsula lentis), and consists of a 

 softer cortical substance (Substantia corticalis), and a dense central part, the 

 nucleus of the lens (Nucleus lentis) . The capsule is thickest on the anterior surface, 

 and here it is lined by a layer of flat polygonal cells, the epithelium of the lens 

 (Epithelium lentis) . The lens substance, when hardened, is seen to consist of con- 

 centric laminee arranged somewhat like the layers of an onion, and united by an 

 amorphous cement substance. The laminae consists of lens fibers (Fibrse lentis), 

 hexagonal in section, and of very different lengths. Faint lines radiate from the 

 poles and indicate the edges of layers of cement substance which unite the groups 

 of lens fibers. These lines, the radii lentis, are three in number in the foetus and 

 new-born, and form with each other angles of 120 degrees. On the anterior surface 

 one is directed upward from the pole and the other two diverge downward; on the 

 posterior surface one is directed downward and the others diverge upward. The 

 developed lens has neither vessels nor nerves. 



In the foetus the lens is nearly globular, and is soft and pink in color. During part of foetal 

 hfe it is surrounded by a vascular network, the tunica vasculosa lentis. This is derived chiefly 

 from a temporary vessel, the hyaloid artery, which is a continuation forward of the arteria cen- 

 trahs retinae through the hyaloid canal that traverses the vitreous body. In old age the lens tends 

 to lose its elasticity and transparency; it also becomes flatter and the nucleus especially grows 

 denser. 



The Ear 



The ear or organ of hearing (Organon auditus) consists of three natural divi- 

 sions — external, middle, and internal. 



THE EXTERNAL EAR 



The external ear (Auris externa) comprises — (1) the aturicula, a funnel-like 

 organ which collects the sound waves, together with its muscles; and (2) the 

 external acoustic meatus, which conveys these waves to the tympanic membrane, 

 the partition which separates the canal from the cavity of the middle ear. 



The auricula or pinna is attached by its base around the external acoustic 

 process in such a manner as to be freely movable. In the following description it 

 will be assumed that the opening is directed outward and that the long axis is 

 practically vertical. It has two surfaces, two borders, a base, and an apex. The 

 convex surface or dorsum (Dorsum auriculae) faces medially and is widest in its 

 middle part; its lower part is almost circular in curvature, while above it narrows 

 and flattens. The concave surface (Scapha) is the reverse of the dorsum; it 

 presents several ridges which subside toward the apex. The anterior border is 

 sinuous; it is largely convex, but becomes concave near the apex. It divides below 

 into two diverging parts (Crura helicis). The posterior border is convex. The 

 apex is flattened, pointed, and curved a little forward. The base is strongly convex. 

 It is attached to the external acoustic process of the petrous temporal bone, and 

 around this there is a quantity of fat. The parotid gland overlaps it below and 

 laterally. The structure of the external ear comprises a framework of cartilages 



