268 STUDIES IN PHYSIOLOGY 



by impulses sent out from the brain and spinal cord. This 

 part of our nervous machinery is called the sympathetic 

 system because its cells and fibers make the various involun- 

 tary muscles of the body work in harmony or sympathy 

 with each other. 



5. THE NERVOUS SYSTEM OF A FROG 



Reason for studying Frog's Brain. We have stated that 

 the human brain has a very complicated structure. This 

 is less true of the central nervous system of other animals. 

 For, in comparing the nervous systems of various kinds of 

 vertebrates, one finds that in the lower groups the brain is 

 considerably more simple. Such a brain has the frog, and 

 the study of the central nervous system of the frog will help 

 much toward a comprehension of the structure and functions 

 of the human brain. 



The frog's brain, like that of the human being, is a con- 

 tinuation of the spinal cord inclosed within a bony cranium. 

 Three regions may be distinguished: the forebrain, the 

 midbrain, and the hindbrain. 



Forebrain. The forebrain consists principally of two 

 elongated masses called the cer'e-bral hemispheres. Nerve 

 fibers run in from the sensory cells of the nose to the 

 anterior part of these cerebral hemispheres, and carry to 

 the brain the impulses that give to the animal the sense 

 of smell. For this reason these nerve trunks are known 

 as ol-fac'to-ry nerves (Latin olfactus smell), and the portion 

 of the hemispheres with which they connect are called the 

 olfactory lobes. 



Midbrain. Two prominent enlargements cover the top 

 and sides of the midbrain. They are called the op' tic lobes 

 (Greek optikos } relating to sight), from the fact that the 

 large nerves from the eyes enter them. On the ventral 

 surface of the brain these two optic nerves cross each other, 

 the nerve fibers from the right eye passing to the left optic 



