186 GENERAL ZOOLOGY 



impulses may be carried to the brain by ascending tracts and 

 back by descending tracts to spinal motor neurons, the nature 

 of the response being altered or controlled as a result of this 

 special routing. Responses, under such conditions show the 

 effect of past experience, as for example in conditioned reflexes. 

 Shining a light into the eye causes a contraction of the pupil, but 

 ringing a bell does not. If a bell is rung each time light is 

 directed into the eye, after a while ringing the bell will cause the 

 pupil to contract without the stimulus of the light. The spinal 

 cord is composed principally of relatively simple reflex arcs and 

 longitudinal tracts that connect these arcs with the brain. 



Consciousness seems to be a function principally of the cere- 

 brum as a whole. The reactions of a person to a pin prick may 

 be the same when he is asleep as when he is awake, but when 

 asleep the stimulus is not perceived. Intelligence, memory, 

 interpretation of sensations are all general functions of the cere- 

 brum. Localized areas, such as the "motor areas," are con- 

 cerned with the control of individual muscle movements. Thus, 

 if the area of the cerebrum controlling the movement of the foot 

 and leg muscles is stimulated, the response is a coordinated 

 movement, entirely unlike the uncoordinated movements 

 produced by directly stimulating the peripheral nerves supplying 

 these parts. There are many such "centers" in the cerebrum 

 and also in other parts of the brain. The location of such "cen- 

 ters" in the brain is the principal difference between the brain 

 and the cord. 



Sense Organs. — Sense organs are specialized in their sus- 

 ceptibility to different kinds of stimulation and are classified 

 accordingly. Since the matter of sensation also enters into the 

 problem, more is known of human sense organs than of those of 

 other animals, where one is limited to studying the responses — ■ 

 muscular or glandular activity — and where one can know but 

 little of the sensation experienced by the animal. The best we 

 can do is to compare their responses with our own under similar 

 conditions. Similarity in the structure of the sense organs of 

 vertebrates is fair presumptive evidence of similar function and 

 of similar, though not necessarily identical, sensations. A classi- 

 fication of sense organs for animals generally is patterned on a 

 classification of human sense organs. These include the follow- 

 ing types of receptors: (1) tactile, (2) pain, (3) warmth, (4) cold, 



