250 THE HUMAN BODY 



edges of the iris are muscle-fibers which by their contraction en- 

 large the pupil. Both sets of muscles are under the control of 

 autonomic nerves. Those to the constrictor-fibers reach the eye 

 by way of the third cranial nerve and belong to the cranial au- 

 tonomic system; those innervating the dilator-fibers enter by way 

 of the ophthalmic branch of the fifth nerve. These latter fibers be- 

 long to the thoracico-lumbar autonomic system and have a rather 

 tortuous connection with the central nervous system. The path- 

 way starts in the upper thoracic region of the spinal cord where the 

 cell-body of the preganglionic neuron lies. The axon of this neuron 

 passes out from the cord to the sympathetic chain and in this chain 

 up the neck to the superior cervical ganglion at the base of the 

 skull. Here the preganglionic neuron terminates in connection with 

 a post-ganglionic. The axon of the latter passes to the fifth nerve 

 and along this to its termination in the pupillo-dilator muscle. 



The iris contains pigment which is yellow, or of lighter or darker 

 brown, according to the color of the eye, and more or less abun- 

 dant according as the eye is black, brown, or gray. In blue eyes 

 the pigment is confined to the deeper layers, and modified in tint 

 by light absorption in the anterior colorless strata through which 

 the light passes. 



The third coat of the eye, the retina, 15, is its essential portion, 

 being the part in which the light produces those changes that give 

 rise to impulses in the optic nerve. It is a still less complete en- 

 velope than the choroid, extending forwards only as far as the 

 commencement of the ciliary processes, at least in its typical 

 form. It is extremely soft and delicate; and, when fresh, trans- 

 parent. Usually when an eye is opened the retina is colorless; 

 but when the eye has been cut open in faint yellow light and the 

 exposed retina quickly examined in white light it is seen to be 

 purple. The coloring substance (visual purple] very rapidly 

 bleaches when a dead eye is exposed to daylight. On the front or 

 inner surface of the human retina two special areas can be dis- 

 tinguished in a fresh eye. One is the point of entry of the optic 

 nerve, 16, the fibers of which, penetrating the sclerotic and cho- 

 roid, spread out in the retina. At this place the retina is whiter 

 than elsewhere and presents an elevation, the optic disk. The 

 other peculiar region is the fovea centralis, 18, which lies nearly 

 at the posterior end of the axis of the eyeball and therefore out- 



