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 during life 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 (Fibree 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 laminz arranged somewhat like the layers of an onion, and united by an 
amorphous cement substance. The lamin consists of lens fibers (Fibre 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 fcetus 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 
life it is surrounded by a vascular network, the tunica vasculosa lentis. This is derived chiefly 
from a temporary ve sssel, the hyaloid artery, which is a continuation forward of the arteria cen- 
tralis retinee 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- 
external, middle, and internal. 
sions 
THE EXTERNAL EAR 
The external ear (Auris externa) comprises—(1) the auricula, 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 auriculee) faces medially and is widest im its 
middle part; its lower part is almost circular in curvature, while above it narrows 
and flattens. The concave surface (Seapha) 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 
