386 



THE NERVOTTS SYSTEM 



cations from one organ to another, or from one part of an 

 organ to another, be made thi-ough the brain and cord. 



If we enlarge our idea of this 

 arrangement and conceive of 

 the brain and cord as not only 

 switching messages from one 

 part of the body to another, 

 but also of being able to modify 

 the character of these messages, 

 we shall have a true picture of 

 the manner in which the brain 

 and cord control and harmonize 

 the actions of all parts of our 

 bodies. 



The nervous system of the 

 frog. — A dissection of the ner- 

 vous system of the frog is easily 

 made, and since it is essentially 

 similar in kind to ours, a study 

 of this system forms a good in- 

 troduction to the study of the 

 human system. 



As in our body, the brain is 

 inclosed in the skull, and is con- 

 tinued downward as the spinal 

 cord through the spinal cavities 

 of the vertebra;. In this posi- 

 tion the system is well pro- 

 tected from external pressure. 

 If this bony covering be carefully removed so as not to 

 injure the contents or its connections, a structure, such as 

 shown in Figure 178 is seen. 



Fig. 178 — Nervous system of frog; 



a, cerebral hemispheres (fore- 

 brain) 6, olfactory lobes; c, eyes; 



d, midbrain; e, optic lobes; /, 

 hindbrain (cerebellum); gr, 

 medulla; h, i, spinal cord; / to 



Xf first to tenth pair of cranial 

 nerves; 1 to 10, spinal nerves. 



