390 THE NEBVOTJS SYSTEM 



of spinal nerves whose fibers are distributed to various 

 parts of the body. , Figure 178 illustrates the method of 

 branching of these nerves. 



Comparison of the frog's spinal cord and brain with that 

 of higher forms. — The spinal cord and brain are to be con- 

 sidered as one structiire, the brain lobes and swellings being 

 simply modifications of the more uniform spinal cord. 

 These modifications become much more marked in the 

 higher forms, and instead of lying directly behind one an- 

 other the increase in size and the lack of space compels these 

 brain lobes to become more or less twisted and doubled upon 

 one another. If we keep the parts of the frog's brain in mind, 

 however, we shall have no difficulty in recognizing the 

 homologous parts of the human central system, as, aside 

 from the folding and variation in size, they are almost iden- 

 tical, part for part. 



Let us now examine the human brain with this com- 

 parison in mind, and see if we can identify the structures 

 found there.^ 



The htmian brain. — The brain of man is inclosed in that 

 part of the skull called the cranium, and it fills this part 

 completely. At the base of this cranium is an opening 

 called the foramen magnum through which it is continued 

 downward as the spinal cord. Various apertures in this 

 bony box permit the entrance and escape of the cranial 

 nerves. 



Brain membranes. — When the bony plates of the cra- 

 nium are removed, the brain is found to be covered with 

 three membranes. The outermost {the dura mater) has 



' The brain o£ the sheep is easily obtainable, and is so nearly identical 

 with that of man in structure that it serves well for laboratory work. 

 (See Ex. LXIII.) 



