CLASS AMPHIBIA 



369 



spinal cord is short, corresponding to the shortness of the body. J.ike 

 the brain it is inclosed in two membranes — a firm outer protective dura 

 mater and a more delicate inner vascular pia mater. The cerebrospinal 

 system includes 10 pairs of cranial nerves and also 10 pairs of spinal 

 nerves. 



The principal sense organs of the frog are the eyes, the auditory 

 organs, and the olfactory organs. In addition to the upper and lower 

 eyelids there is a third, called the nictitating inemhrane, which is fused 

 with the lower one. The lens is large and nearly spherical and there 

 is little power of accommodation. The auditory organ has been in a 

 general way described in the preceding topic. There are three semi- 

 circular canals. The olfactory epithelium lines cavities just within 

 each of the nostrils. 



ABC 



Fig. 251. — Diagram to illustrate modification of color by ameboid chromatophores. 

 A, pseudopodia fully extended; B, partially extended; C, contracted. In A the color 

 present in deeper layers of the skin, represented by crosslining, is obscured, and in C it 

 prevails. 



399. Behavior. — Endeavors have been made to determine the 

 function of the different regions of the frog's brain by the removal of 

 one after another in the living frog. It has been found that removal 

 of the cerebral hemispheres, together with the olfactory lobes, seems 

 to have little effect. When the mid-brain is removed the frog loses 

 its power of spontaneous movement, which is clearly connected with 

 the loss of sight. Also the spinal cord becomes more irritable, which 

 shows that the destruction of the mid-brain and the loss of sight have 

 removed a control which was necessary to nervous equilibrium. The 

 very small size of the cerebellum seems to show that it has no important 

 function. After all of the brain is removed except the medulla the 

 animal still continues to breathe, will snap at food brought in contact 

 with its jaws, is able to leap, swim, and right itself when placed upon 

 its back. Destruction of the medulla, however, results in death. This 

 clearly shows that the vital centers are lodged in the medulla and that 

 the actions of the frog are very largely reflex in character. 



Since it is true that the activities of the frog are mainly reflex it is 

 also clear that they are governed largely by instinct. The fact that 



