THE EXTERNAL EAR 747 



more strongly curved than the anterior. It rests in the fossa of the vitreous body. 

 The central points of the surfaces are the anterior and posterior poles (Polus 

 anterior et posterior lentis), and the line which connects them is the axis of the 

 lens (xVxis lentis). 



The transverse diameter of the lens is about 2 cm., the vertical diameter is slightly smaller, 

 and the axis measures about 13 mm. The radius of curvature of the anterior surface is 13.5 mm., 

 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 (of Zinn) or suspensory ligament of the lens (Fig. 552) 

 consists of delicate fibers (Fibrs 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 inclosed by a structureless, 

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

 softer substantia corticalis, and a dense central part, the nucleus lentis. The cap- 

 sule is thickest on the anterior surface, and here it is lined by a layer of flat poly- 

 gonal cells, the epithelium of the lens capsule. The lens substance, when hardened, 

 is seen to consist of concentric laminae arranged somewhat like the layers of an 

 onion, and united by an amorphous cement substance. The laminge consist of 

 lens fibers, 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 fcetus the lens is nearly globular, and is soft and pinkish 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 vessel, the hyaloid artery, which is a continuation forward of the arteria 

 centrahs 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 

 divisions — external, middle, and internal. 



THE EXTERNAL EAR 



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

 organ w^hieh collects the sound waves, together with its muscles, and (2) the 

 external auditory canal, 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 auditory 

 canal 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 auricula?) faces inward 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 tow^ard the apex. The anterior border is sinuous; it is largely 

 convex, but becomes concave near the apex. It divides below into two diverging 



