A STUDY OF THE NERVOUS SYSTEM 273 



search of insects, fish, or smaller frogs, and varies his pro- 

 cedure with each species of victim. The physiologist can- 

 not, by manipulating him, elicit croaking, crawling up a 

 board, swimming, or stopping at will. His conduct has 

 become incalculable we can no longer foretell it exactly. 

 Effort to escape is his dominant reaction, but he may do 

 anything else, even swell up and become perfectly passive 

 in our hands." 



Summary. From all these experiments we conclude 

 (1) that the cells and fibers of the spinal cord are able to 

 direct reflex movements of a defensive kind, without any 

 help from the brain ; (2) that the hind- and midbrains con- 

 trol the processes of locomotion, swallowing, breathing, and 

 croaking; and (3) that all the voluntary actions of the 

 animal are governed by the f orebrain. It is probable, too, 

 that all the messages which come in from the eyes, ears, 

 nose, and skin reach the forebrain before the frog experi- 

 ences any kind of sensation. 



6. ANATOMY OF THE HUMAN BRAIN 



Protection for the Brain. The human brain is an exceed- 

 ingly delicate mechanism, and would be liable to frequent 

 injury were it not well protected. In the first place, the 

 thick growth of hair and the loose, tough scalp form out- 

 side coverings for the head, which help to deaden the force 

 of possible blows. Again, the arched form of the cranium 

 and its several layers of bone tissue (two layers of hard 

 bone separated by spongy bone) give to this brain case 

 the greatest possible elasticity and strength. In the third 

 place, the springiness of the curved spinal column and of 

 the arched instep tends to keep the delicate structures 

 within the skull from being jarred. And, finally, the brain 

 itself is inclosed by the dura mater, arachnoid, and pia 

 mater, which are continuous with the corresponding mem- 

 branes about the cord. 



