708 EVOLUTION OF ANIMALS Part V 



Autonomic Nerves. The autonomic nerves regulate involuntary action, 

 routine functions such as those of muscles in the alimentary canal, blood 

 vessels, and glands. Fibers of the autonomic nerve cells enter and leave the cord 

 and brain in the cranial and spinal nerves. 



The whole autonomic system was formerly called the sympathetic system. 

 That term is now commonly used for the chains of nerves and ganglia which 

 serve the viscera. They lie on either side of the thoracic and lumbar vertebrae. 

 Autonomic is the word used for the entire system with reference to its in- 

 voluntary nature (Chap. 16). 



Sense Organs. The sense organs are described in Chapter 17 and only 

 particular applications to frogs will be given here. 



Sense organs or receptors are cells or groups of cells whose content is 

 changed or stimulated by particular conditions in the environment. Familiar 

 ones are the eye and ear; less known are the receptors of cold and heat in the 

 skin. 



The frog's skin is sensitive to touch, to cold and heat, to pain, to acids and 

 other irritants in each case through different sensory cells. To some degree, 

 frogs taste through their skins. They can also detect odors under water as well 

 as in the air. The lateral line organs of balance that are well developed in tad- 

 poles are absent in most species of adult frogs. 



The frog's eye has some markedly fishlike characters. It will not accom- 

 modate, that is, the shape of the lens cannot be changed nor can it be moved 

 nearer and farther from the retina to any such degree as the human lens. In 

 the air, frogs are nearsighted; in the water, they are farsighted; in either 

 medium they see moving objects best. Because their eyes are located so far 

 to the sides of the head, frogs cannot easily use both eyes on an object directly 

 in front of them. 



Frogs have a well-developed sense of hearing. They respond to croaks 

 heard in the distance, also to simulated croaks. Anyone who has disturbed a 

 populous spring frog pond knows the sudden silence that falls upon it. Then 

 after a waiting time of complete quiet, one frog raises a solitary voice and, as 

 if that were a signal, other frogs one after another begin to call. One of the 

 sure proofs that a frog hears is the quickening of its throat movements when 

 a bell is rung in a nearby room. 



Frogs have a sense of balance. This is located in the semicircular canals 

 associated with the inner ear (Chapter 17). 



Reproduction. Female Organs. In winter, the ovaries are the most con- 

 spicuous objects in the body cavity. The eggs are then absorbing food from 

 the blood and approaching full size. Beneath the membrane of each egg a 

 layer of black pigment partially surrounds the yolk. After the breeding season, 

 the ovaries are a small fraction of their former size with the eggs of another 

 season hardly visible. 



