THE FROG 45 



as a gland. From their characteristic form these structures in the 

 skin of the frog are known as flask-glands. 



The dermis is much thicker than the epidermis, and is composed 

 of connective tissue. Most of the fibres in it run parallel with the 

 surface of the skin, but a certain number of strands of them run at 

 right angles to the remainder. Unlike the epidermis the dermis 

 has a supply of blood-vessels. They are much more plentiful than 

 in other animals, for in the frog the skin, in addition to its other 

 functions, plays a large part in the breathing. Immediately beneath 

 the Malpighian layer are a number of cells deeply impregnated 

 with granules of pigment, and another layer of more sparsely 

 scattered pigment cells is situated deeper down. It is to the presence 

 of these pigment cells that the skin owes its colour, and to their power 

 of altering their shape, the possibility of altering the colour within 

 limits. The dermis is also supplied with nerves which end just 

 beneath the epidermis, or partly in it, in sense corpuscles, as they are 

 termed. These are especially abundant in regions of great sensi- 

 bility, such as the tongue. 



The principal function of the skin is protection, and we find that 

 it presents a tough surface to the outer world, and so guards the 

 underlying parts from injury. The blood-vessels do not come right 

 to the surface, so that a superficial wound does not result in a loss 

 of blood. The skin is also protective in another sense, for its colour 

 pattern and the power of altering it enables the animal to harmonise 

 with its surroundings, and so avoid detection by its enemies. The 

 second function of the skin in the frog is respiration, and so we find 

 a large supply of blood-vessels in it, and these are not shut off from 

 the air by such a thick layer of horny cells as we find in many 

 animals. Then again this process is aided by the secretion of the 

 cutaneous glands, which keeps the skin always moist, and so in a 

 condition in which the exchange of gases can most readily take place. 

 It is not improbable that this secretion may also help the animal to 

 rid itself of certain obnoxious substances. 



