438 ZOOLOGY FOR MEDICAL STUDENTS chap. 



nervous connexions. In particular cases the tactile function of hairs 

 may become developed to a very high degree as may be demonstrated 

 by the intolerable sensation produced by bringing a vibrating tuning 

 fork into contact with one of the hairs of the moustache in man, or one 

 of the " whiskers " of a cat. 



The hairy coating shows various divergences from the normal. In 

 aquatic mammals it tends to become reduced. In the whales it has 

 disappeared altogether except for a few vestiges in the neighbourhood 

 of the mouthj its heat-retaining function being taken over by the greatly 

 developed layer of fat in the dermis (blubber) which possesses the im- 

 portant advantage over hair of having a lower specific gravity. In 

 man the hairy coat has degenerated in correlation with the adoption of 

 artificial heat-retaining coverings. In a few mammals some of the 

 individual hairs become enormously enlarged so as to form defensive 

 weapons {Echidna, Hedgehog, Porcupine). 



The skin of the normal, terrestrial, mammal is provided with an 

 abundance of epidermal glands and these are specialized in relation to 

 three functions. 



(i) In relation to the heat-retaining function each individual hair 

 is kept dressed with the secretion of sebaceous glands opening on the 

 inner surface of the hair-follicle. 



(2) In relation to homoiothermy — the constancy of body tempera- 

 ture — certain glands are specialized for the purpose of counteracting to 

 the needful extent the heat-retaining properties of the hair, according as 

 the temperature gradient between the body and the external air varies 

 in steepness — whether owing on the one hand to changing intensity of 

 metabolism and therefore of heat-production within the body, or on the 

 other to changes in external temperature. These glands are the sweat- 

 glands : they produce a watery secretion which diffuses over the surface 

 of the body and by its evaporation produces a cooling effect. In aquatic 

 mammals the sweat-glands naturally tend to disappear, and the same 

 has happened in a few terrestrial mammals. In the Dog for example 

 they have disappeared for the most part except on the soles of the feet, 

 and their cooling function has to be carried out by other means, by 

 increasing the activity of breathing — " panting " causing a great increase 

 in the amount of air drawn into the lungs and sent out again with a load 

 of surplus heat. 



(3) In relation to the function of nourishing the progeny certain 

 epidermal glands (mammary glands) have become specialized for the 

 secretion of milk. These are crowded together into paired masses 

 (mammae) on the ventral surface of the body, the number of which 



