THE FROG AND VERTEBRATES IN GENERAL 221 



on a number of regularly coordinated movements. It is clear, 

 therefore, that such actions do not necessarily involve the an- 

 terior parts of the brain. Responses of this type which do not 

 involve the higher parts of the brain are known as reflex 

 actions. The hind-brain and the anterior end of the spinal cord 

 are particularly adapted for the more involved types of reflex 

 action. There are many reflex actions of a lower grade which are 

 located even farther posteriorly in the spinal cord. 



Good examples of reflex actions may be seen in many of our 

 own reactions. If, for example, a finger touches a hot surface it is 

 immediately jerked away. This is a reflex action and as such does 

 not involve definite thought on our part. In such an action the 

 impulse from the sensory cells in the skin enters the central nerv- 

 ous system through the dorsal root of a spinal nerve. In the 

 spinal cord the impulse is relayed to the proper motor cells and, 

 leaving the spinal cord through a ventral root, reaches the connect- 

 ing muscles and incites the appropriate response. Or, in learning 

 to walk, each step involves thought, but after a time the actions 

 become largely reflex in their nature, and the higher brain centers 

 are thereby freed for other more important functions. Thus it 

 is with many other similar actions which are first acquired by 

 conscious effort. (W. f. 143.) 



TEXTBOOK REFERENCES 

 Woodruff, pp. 132-217 ; 218-280. 



Curtis and Guthrie, pp. 38-129 ; 397-455. 



Guyer, pp. 49-188 ; 407-477. 



Hegner, pp. 464-514. 



Newman, pp. 302-370 ; 397-410 ; 437-457. 



Shull, pp. 89-141 ; 142-212. 



GENERAL REFERENCES 



Gadow. " Amphibia and Reptiles," in the Cambridge Natural History 



(Macmillan). 

 Holmes. Biology of the Frog. 4th ed. (Macmillan). 

 Howell. Textbook of Physiology. 11th ed. (Saunders). 

 Kingsley. Comparative Anatomy of Vertebrates (Blakiston). 

 McEwen. A Textbook of Vertebrate Embryology (Holt). 

 Noble. The Biology of the Amphibia (McGraw-Hill). 

 Walter. Biology of the Vertebrates (Macmillan). 



