206 HOW BODY CONTROL IS BROUGHT ABOUT 



most delicate layer in the entire body. Despite the fact that the 

 retina is less than -^-^ of an inch in thickness, there are several layers 

 of cells in its composition. The optic nerve enters the eye from 

 behind and spreads out over the surface of the retina. Its finest 

 fibers are ultimately connected with numerous elongated cells, 

 which are stimulated by light. The retina is dark purple in color, 

 this color being due to a layer of cells next to the choroid coat. 

 This accounts for the black appearance of the pupil of the eye, 

 when we look through it into the darkened space within the eye- 

 ball. The retina acts as the sensitized plate in the camera, for on 

 it are received the impressions which are transformed and sent to 

 the brain and result in sensations of sight. The eye, like the 

 camera, has a lens. This lens is formed of transparent, elastic 

 material. It is directly behind the iris and is attached to the 

 choroid coat by means of delicate ligaments. In front of the lens 

 is a small cavity filled with a watery fluid, the a'queous humor, 

 while behind it is the main cavity of the eye, filled with a trans- 

 parent, almost jelly-like, vifreous humor. The lens itself is elastic. 

 This circumstance permits a change of form and, in consequence, 

 a change of focus upon the retina of the lens. By means of this 

 change in form, or accommodation, we are able to see both near 

 and distant objects. 



Summary. — Plants and animals respond to stimuli of the same 

 nature : light, heat, gravity, etc. These responses are called 

 tropisms. But the mechanism by which the response is brought 

 about is not the same. Plants have no sense organs, although 

 parts of their bodies are more sensitive to seme kinds of stimuli than 

 other parts. Many of their responses are growth responses. 

 But nearly all animals have special sense organs and nerves which 

 stimulate muscles and thus bring about movement. 



In all but the lowest animals the simple reflex arc is the most 

 typical nervous mechanism. In this we have a sensory neuron 

 sending its axon back to a motor cell, stimulating it through a 

 synapse, thus causing a movement by means of the motor endings 

 in the muscle. Thus a reflex action is brought about. 



The vertebrate animals have the most complicated nervous 

 system and sense organs. They are provided with a central nervous 

 system : brain and spinal cord, protected by bony coverings; — 



