66 General Biology 



forms of animals a definite nervous system does this work and permits 

 a coordination of activities in different parts of the body. For example : 

 In order to leap when danger threatens, the frog must be able to send 

 the necessary nervous impulses to both hind legs at one time, for if 

 only one leg should get an impulse, the frog would fall over on one 

 side instead of propelling its body for some distance ahead. 



There is also another function performed by the nervous system. 

 That is the accumulation of the effects of experiences which the animal 

 in question has had, so that it may profit by the memory of these ex- 

 periences in new situations. When this ability is highly developed, 

 as in man, we speak of it as reasoning or intelligence. However, when 

 the animal only remembers, let us say, a physical punishment for a 

 given act, and by sheer association of the punishment and the act, ceases 

 to perform the act which brought about the punishment, such an asso- 

 ciation is not known as intelligence, but as association memory. 



Practically all parts of the body have nerves running to them. 

 There are three closely associated divisions in the nervous system, 

 (Fig. 17) known as : 



1. The central, consisting of brain and spinal cord. 



2. The peripheral, consisting of cerebral and spinal nerves, and 



3. The sympathetic, supplying non-striated muscles. 



THE CENTRAL NERVOUS SYSTEM 



As in all vertebrates, the brain and spinal cord are on the dorsal 

 side of the animal, being contained within a bony case known as the 

 skull and neural canal. It will be noted that beginning at the anterior 

 end, the brain consists of quite distinct parts, namely, the olfactory lobes, 

 the cerebral hemispheres, the two large optic lobes, a well developed 

 mid brain, a small cerebellum, and a broadening of the spinal cord 

 itself, called the medulla oblongata. From the ventral surface, we may 

 see in addition the crossing from one side to the other of the optic nerves, 

 known as the optic chiasma. 



A small process directly behind the optic chiasma, called the in- 

 fundibulum, ( ) ends in another small body, 



the pituitary body, ( ) or hypophysis ( ). 



On the dorsal side of the mid brain is found the pineal gland, 

 ( ) or epiphysis ( ), already 



mentioned as a rudimentary organ which, in some forms of reptiles, 

 forms a dorsal median eye. The cerebrum and olfactory lobes 

 ( ) together constitute the fore brain, the 



optic lobes form the mid brain, the cerebellum and medulla form the 

 hind brain. 



It is not clear what functions each part of the frog's brain can per- 

 form. From various experiments, however, it is known that the frog 

 loses the power of spontaneous movement if the mid brain and cerebral 



