THE INTEGUMENTARY SYSTEM 



179 



Italians are more liable than lighter stocks to rickets, which is a sunlight 

 deficiency disease. So it may be that in high latitudes, in a wooded coun- 

 try or one that has much cloud and fog, a fair skin that is still able to tan 

 may have a selective value and be accounted for on Darwinian principles. 

 Skin color plays queer tricks. Any two parents, even two Negroes, 

 may have an albino child; two dark-haired parents may somehow miss 

 with the brown pigment and have red-haired offspring. The most that 

 anyone can say is that "Nordic" man probably began as a mutant from 

 a dark stock. Possibly, after the mutation appeared, it was admired and 

 selective mating kept it to the fore. 



ASSOC I ATI OM 

 NEURONE 



EFFERENT NERVE 



• SWEAT GLAND 



iCAPILI-ARIES 



Fig. 136. — A diagram illustrating the nervous mechanism of temperature regulation 

 in man. The quantity of secretion of tubular glands (and consequently the amount 

 of sweat which may evaporate to cool the body) depends upon the quantity of blood in 

 the capillaries associated with the glands and dermal papillae. Through a reflex arc the 

 circulation is regulated by the temperature of the skin. (Redrawn after Hough and 

 Sedgwick.) 



CUTANEOUS GLANDS 



Since among invertebrates most glands are unicellular, it has generally 

 been assumed that the multicellular glands of vertebrates have evolved 

 from such beginnings, an increase in the size of the secreting cells tending 

 to carry them into the underlying corium, where together with other 

 epidermal cells, they become multicellular organs. 



Be this as it may, cutaneous glands develop, much as hairs do, from 

 solid cords, which are proliferated from the stratum germinativum and 

 grow downward into the underlying corium. The lumen of the gland 

 forms later, to connect with the exterior, and the gland anlage differen- 



