66 GENERAL BIOLOGY 



movements. In the higher forms of animals a definite nervous system 

 does this work and permits a co-ordination of activities in different 

 parts of the body. For example : In order to leap when danger threat- 

 ens, 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 his body for 

 some distance ahead. 



There is also another function the nervous system has to perform, 

 and that is the accumulation of the effects of experiences which the ani- 

 mal in question has had, so that such animal may profit by the memory 

 of these experiences in new situations. When this ability is highly de- 

 veloped, we speak of it as reasoning or intelligence, whereas when the 

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

 performed 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 association 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 of the 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 an- 

 terior end, the brain consists of quite distinct parts, namely, the olfac- 

 tory 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 6f the optic 

 nerves, known as the optic chiasma. 



A small process directly behind the optic chiasma called the infundi- 

 bulum ( ) 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 the reptiles, 

 forms a dorsal median eye. The cerebrum and optic lobes (thalamen- 

 cephalon) ( ) 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- 



