COORDINATION AND BEHAVIOR 



547 



the legs, or difficulty in talking because of 

 lack of coordination in the muscles moving 

 the tongue and jaw. 



The cerebrum governs all mental activ- 

 ities: it is the seat of consciousness, asso- 

 ciative memory, reasoning, and intelligence; 

 it originates voluntary acts and influences 



many reflex acts. Laughing, weeping, urinat- 

 ing, defecating, and many other acts are 

 examples of the latter. The brain, with its 

 various centers of activity, is to the nervous 

 system what the wiring arrangements and 

 switchboards are to the central office of a 

 telephone system. 



Muscle movements 

 and muscle sense 

 (motor) 



Head and eyes 

 Motor speech 



Frontal lobe 

 Temporal lobe 



~ Central sulcus 



Skin sensations 



(sensory) 



Reading 



Sight 

 Hearing 



Cerebellum 



Figure 394. Diagram of the left side of the human brain to show the main lobes and the 

 areas concerned with certain functions. The cerebrum, which reaches its greatest development 

 in man, is by far the largest part of the brain. 



The motor areas are located in front of 

 the central sulcus, a groove, as indicated in 

 Fig. 394, and the sensory areas are behind it. 

 These sensory areas include those of sight 

 (visual), hearing (auditory), and skin sen- 

 sations. The regions surrounding these are 

 association areas, which are concerned with 

 the higher psychic activities. 



Sense organs and sensations 



The principal sense organs of the frog and 

 other vertebrates are the nose, eyes, and ears. 

 In some of the lower vertebrates, conspicu- 

 ous lateral line organs tell of water move- 

 ments and the pressure of current against 

 the body (Fig. 253). Sensory cells of several 

 types are also located in various parts of the 

 body. Sensations result when receptors are 

 stimulated, and impulses are transmitted to 

 the brain by afferent nerves. 



The "sensations" of lower animals must 



be interpreted on the basis of what we 

 know regarding those of man. They may be 

 grouped according to the types of receptors 

 involved. Thus we may recognize mechano- 

 receptors, such as pressure on the skin; 

 chemoreceptors, such as smell and taste; and 

 radioreceptors, such as those for light, heat, 

 and cold, which are excited by radiant en- 

 ergy, etc. 



The sense of touch results from the stim- 

 ulation of tactile receptors by contact with 

 objects. In man, the papillae of the dermis, 

 especially of the palmar surface of the hands 

 and fingers, which contain tactile corpuscles, 

 give the skin its sense of touch. The sense of 

 hunger is due to receptors in the stomach 

 wall that are stimulated by contractions of 

 this organ when empty. The sensation of 

 appetite may be due to similar receptors, 

 but is probably aroused by the nerves of 

 taste and smell. Thirst is a sensation result- 



