AMPHIBIA 



275 



on the dorsal surfaces of the extremities, but not to any extent on 

 the abdomen, breast and ventral surface of the extremities, and is 

 absent in the feet. 



Pigment of the Skin. — The greater quantity of the pigment cells 

 of the skin is found in the cutis. These cells are called Chromato- 

 phores. They play an important part in bringing about the well- 

 known changes in the coloring of the skin. These cells have been 

 found to be supplied by nerve fibers. When the nerve fibers were 

 stimulated, the cells were influenced. Blinded, the frog cannot 

 change color. 



fVebs and Folds. — On the under side of the toes are little cushions 

 or pads. The toes are connected together by a web which makes the 

 foot an excellent paddle. Behind the eyes there extend two ridges 

 formed by a thickening of the skin. These are the dorso-lateral 

 dermal plicae or folds. 



Muscle. — The muscles of the frog retain their vitality for a long 

 time after they have been removed from the body, and accordingly 

 they are well adapted for physiological experiments, the large 

 gastrocnemius or calf muscle of the frog being used a great deal in 

 the study of physiological activity. Contraction may be brought 

 about by the application of nervous, thermal, chemical, mechanical 

 or electrical stimuli. The response to stimulus is very rapid in 

 voluntary muscle and is much slower in involuntary muscle. 



Tendon. — Most of the muscles are attached by one or both ends 

 to bones. In some cases the attachment is direct, in others by 

 means of a tendon which is a band of very tough inelastic connective 

 tissue. The outer surface of the muscle is covered by connective 

 tissue or fascia which is more or less elastic. The tendons of many 

 muscles are formed by a continuation of x\\^ fascia., which becomes 

 thicker toward the end of the muscle where it becomes a dense 

 fibrous band. 



Digestive System. (Figure 149.) — The function of food is to 

 afford energy necessary to carry on the various activities of the 

 organism, and to rebuild wastes. In order that food material 

 may be built up into the tissues of the body, it must be rendered 

 soluble, so that it can pass through the inner lining of the alimentary 

 canal into the blood and lymph, and from these fluids through the 

 walls of the cells in the different parts of the body. The frog does 

 not chew the food taken into the mouth but swallows it down the 



