SUBPHYLUM VERTEBRATA 



339 



or more convex. It is inclosed in a thin fibrous capsule which is attached 

 all around by a suspensory ligament to a body known as the ciliary body, 

 from which also the iris extends and which contains a muscle known 

 as the ciliary muscle, in the form of a complete ring. All of these belong 

 to the choroid layer. The tendency of the eyeball to maintain a globular 

 form causes a tension on the lens capsule which serves to flatten the 

 lens and to focus the eye for distance. When it is desired to focus on a 

 near object, the ciliary muscle contracts. This pulls the margin of the 

 suspensory ligament inward or toward the pupil, releases the tension 

 of the capsule, and permits the lens to thicken or become more convex. 

 Thus focusing on a distant object is not a muscular act, but focusing 

 on a near object is. As an individual grows older this elasticity of the 



ConJuncfi\/a 



Posterior, 

 chamber 



■Sclerotic layer 



Choroid layer 

 Retinal layer 



Pigmerrt 

 layer 



Fovea 

 centralis 



Optic 

 nerve. 



Rectus 

 muscle 



Somewhat diagrammatic section of the human eye. 



lens becomes less and less, and thus, while old people with normal eyes 

 find no serious difficulty in seeing things at a distance, they have to put 

 on glasses to assist the eye in focusing on near objects and to enable 

 them to read. 



Accessory organs of sight are the lids, which protect the eye; the 

 lachrymal glands, which secrete a fluid that bathes the free surface of 

 the eyeball and prevents it from drying; and several muscles, which 

 move the eyeball and point it in various directions. 



356. Reproductive System. — All vertebrates are diecious, except 

 the lowest group, the hagfishes. In the male the essential organs are the 

 testes and the vas deferens; in the female they are the ovary and 

 the oviduct. There may be present in the female, along the oviduct, 

 glands which add albumen, shell, and pigment to the eggs, while in the 

 male sex there may also be accessory organs of copulation. In most 



