270 



THE HUMAN MECHANISM 



We shall choose for study the case of a single animal, the 

 frog, the anatomical structure of whose brain has been given 

 in this chapter. The phenomena shown by the frog are, 

 however, as far as we shall describe them, in general true 

 of higher vertebrate animals. 



We shall therefore study (1) the behavior of a frog whose 

 brain has been destroyed, that is, a frog which possesses no 



part of its central 

 nervous system ex- 

 cept the spinal 

 cord ; (2) the be- 

 havior of a frog 

 with spinal cord 

 and bulb intact, 

 but destitute of 

 midbrain, 'tween- 

 brain, and cere- 

 brum ; (3) the 

 behavior of a frog 

 with spinal cord, 

 bulb, midbrain, and 

 'tweenbrain, but 

 destitute of the 

 cerebrum. 



The behavior of 

 these incomplete 

 animals will each 

 be compared with that of a normal frog, which, of course, 

 possesses a complete nervous system. 



5. The behavior of a brainless frog ; that is, a frog which 

 possesses of its nervous system only the spinal cord. Such 

 a frog can carry out only reflex actions of a comparatively 

 simple character. It lies flat upon its belly and, like the 

 normal frog, bends its hind legs under its flank, but does 

 not sit erect by supporting the head and upper trunk on the 



FIG. 110. Transverse section of a convolution of 

 the cerebellum. After Ramon y Cajal 



The figure represents only a few of each kind of nerve 



cells and nerve endings. A, D, E, cells ; B, C, nerve 



endings (synapses) 



