214 THE HUMAN BODY 



Radiating from this ring to the edges of the iris are muscle- 

 fibers which by their contraction enlarge the pupil. Both sets of 

 muscles are under the control of sympathetic nerves. Those to 

 the constrictor-fibers reach the eye by way of the third cranial 

 nerve; those innervating the dilator-fibers enter by way of the 

 ophthalmic branch of the fifth nerve. These latter fibers have a 

 rather tortuous connection with the central nervous system. The 

 pathway starts in the midbrain and passes down the cord to the 

 upper thoracic region 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 back up the neck to the 

 superior cervical ganglion at the base of the skull. Here the 

 preganglionic neuron terminates in connection with a postgan- 

 glionic one. The axon of the latter passes to the fifth nerve and 

 along the latter 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 

 abundant 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 second tunic, 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 fche 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- 



