INTEGUMENTARY SYSTEM 199 



There is certainly no reason why these animals should be 

 incapable of swimming, but it seems as though the fear of the 

 water prevented their doing so just as in the case of a man who 

 has not learnt to swim. 



II. Integumentary System. 



Touch and temperature sense are excluded from this chapter 

 on the physiology of the skin, as they have been already dealt 

 with in the physiology of the senses. There remain to be con- 

 sidered certain functions of the skin which concern the secretion 

 of sweat and sebum, the growth of hair, the shedding of the 

 epidermis, resorption, and transpiration by the skin. 



Perspiration is possible in man, as in horses and sheep, over 

 the entire surface of the body, though certain areas sweat more 

 freely than others ; especially the face (the forehead and ridge 

 of the nose), the axilla, and, as in apes, the palm of the hand and 

 sole of the foot. Among animals the ox sweats much less 

 freely ; carnivora (including dogs and cats) perspire in the sole 

 of the foot only ; goats, rabbits, rats and mice do not at all. 



Von Krause has reckoned the total number of sweat-glands 

 in the human skin at approximately 2\ millions, by no means 

 too high an estimate probably when it is remembered that in 

 the palm of the hand there are 310, and in the sole of the foot 

 300 per square centimetre. 



In both man and animals the amount of sweat secreted 

 varies considerably, being dependent partly on external con- 

 ditions (temperature and moisture of the air), partly on internal 

 stimuli (for example, hot drinks and muscular exertion). Even 

 psychical impulses have to be included in animals as well as in 

 man ; these act through the stimulation of certain efferent nerve- 

 fibres so-called secretory nerves which join the motor nerve- 

 fibres as the anterior roots leave the spinal cord, and are con- 

 nected with certain sweat-centres in the cord. That these 

 sweat-centres do not act solely by direct stimulation but also by 

 reflex stimulation of the skin and sensory nerves has been proved 

 by experiment both for man and animals. 



In the thoracic and abdominal sympathetic nerves, accord- 

 ing to Luchsinger and others, there are secretory-fibres for the 



