334 AN INTRODUCTION TO THE STUDY OF FOSSILS 



nerves of the sympathetic system and their associated ganglia 

 constitute the peripheral nervous system. The sympathetic nerv- 

 ous system, a derivative of the spinal nerves, consists of a 

 pair of nerve-cords extending from the base of the skull to the 

 root of the tail, one upon each side of, and ventral to, the back- 

 bone. From these, branches are given off especially to the 

 blood-vessels and digestive system. At intervals throughout 

 the entire system are enlargements, or ganglia, from which addi- 

 tional nerves arise. The peripheral nerves, composed of fibers 

 of various kinds, divide and subdivide until they reach every 

 minute portion of the body. The spinal cord and the sympa- 

 thetic system are in communication by means of nerves passing 

 from the ganglia of the main sympathetic nerve-cords to the 

 ventral branches of the spinal nerves. 



Organs of special sense. ^ The vertebrate eye is the most 

 highly developed eye in the animal kingdom. It consists of 

 a more or less globular body protected by a tough, fibrous outer 

 layer, the sclerotic coat. Through the anterior transparent 

 portion of this, the cornea, the light passes; next it passes 

 through a watery fluid, the aqueous humor, then through the 

 central black spot, the pupil. The bi-convex tissue forming the 

 lens next concentrates the rays in an image which is projected 

 back through the jelly-like substance, the vitreous humor, 

 upon the inner coat of the eyeball, the retina. In the retina 

 are lodged the ends of the nerves of sight ; these take the sensa- 

 tion, transmitting it to the brain, where the image is perceived. 

 The middle coat of the eye is lined with black, which absorbs 

 all deflected light rays, hence the colorless pupil appears to be 

 black. The colored portion surrounding the pupil, the iris, 

 contains numerous radial and circular muscle fibers, by the con- 

 traction of which alternately the pupil is enlarged and reduced 

 in size, for times of less and greater amount of light respectively. 

 The lens is inclosed in an elastic sac and by means of muscles 

 fastened to the sides of the eyeball may be decreased or in- 

 creased in convexity for seeing objects at a greater or less dis- 



