190 ELEMENTARY PHYSIOLOGY less. 



full of air, it will be found that this air undergoes changes 

 which are similar in kind to those which take place in the 

 air which is inspired into the lungs. That is to say, the 

 air loses oxygen and gains carbonic acid ; it also receives 

 a great quantity of watery vapour, which condenses uj)on 

 the sides of the bag, and may be drawn off by a properly 

 disposed pipe. Further there is a continual loss of heat 

 taking place from the surface of the body. Of these the 

 loss of watery vapour and of heat are of immense im- 

 portance, for it is chiefly by means of variations in their 

 amount from time to time that the temperature of the body 

 is kept nearly constant. But before dealing with these 

 activities of the skin we must understand the main facts 

 as to its structure. 



The skin consists of two parts, an outer layer or 

 epidermis, resting on a deeper layer, the dermis. 

 The skin as a whole is connected to the tissues it covers 

 by a layer of loose fil)rous connective tissue (see Fig. 28), 

 called subcutaneous tissue. This often contains fat, and 

 is the part which is cut through when an animal is 

 skinned. 



The dermis is made up of a dense feltwork of ordinary 

 connective tissue fibres mixed with many elastic fibres 

 and .some connective tissue corpuscles (see Lesson XII.). 

 The surface of the dermis is raised up into little hillocks 

 or elevations known as tlie papillae. Arteries enter the 

 dermis and break up into capillaries which are very close 

 set at its surface and in the papilliB ; thus the dermis 

 is extremely vascular. Nerves also run into the dermis, 

 and passing outwards, form a bi-anching layer of fibres at 

 its junction with the epidermis, and from this layer 

 extremely fine nerve fibrils pass out and between the lower 

 cells of the epidermis. In some parts of the body, some 

 of the branches of the nerves run up into the papillee, 

 where they are connected with sp)ecial nervous structures 

 such as tactile corpuscles and end bulbs. But since 

 these are of importance solely in connection with the 



