368 EXTERNAL INTEGUMENT. 



fluence of some mental shock, has changed its color almost 

 instantaneously ; and if this does not indicate the existence 

 of vitality in the elements of the hair, it proves at least that 

 sudden chemical modifications may be produced in them by 

 certain states of the nerves, acting, either directly, or by 

 means of the blood and vessels. 



The corneous layers thus produced are destined to be 

 separated from the epidermis, and, consequently, to fall into 

 decay, exactly as we have seen in the epithelium of the intestine. 

 In the present instance, however, the decaying layers do not 

 take the form of mucus, or more or less albuminous flakes, but 

 appear as small scales or pellicles, the remains of dried-up 

 cells. The part of the epidermis nearest to the surface, is 

 formed of these layers of fragmentary detritus, just ready to 

 fall: this is what is called the furfuraceous layer, which falls 

 off by slight friction. Pathological causes may sometimes 

 increase this furfuraceous desquamation, and as these epithe- 

 lial remains contain transformed albumen (keratine), sulphur, 

 iron, etc., in such a case the organism suffers an actual loss ; 

 this is the reason that squamous diseases are so dangerous 

 and produce such exhaustion. We have also seen that if the 

 epithelium dissolves into mucus in too large quantities, seri- 

 ous pathological conditions follow, such as bronchitis, and 

 catarrhs in general. It may therefore be said that what is 

 called a pityriasis, or desquamation, in the case of the skin, 

 is a catarrh in that of a mucous surface. 



We have seen that the desquamation of the epidermis does 

 not generally give rise to a fluid like that from the mucous 

 tissue ; there are, however, some less exposed parts of the 

 skin, whose desquamation is less dry, and closely resem- 

 bles the corresponding product of the mucous tissue ; such 

 are the arm-pit, the fatty desquamation of the skin of the 

 gland, and of the inner surface of the prepuce (smegma prce- 

 putii) ; we shall also find, in the sebaceous gkmds, saccular 

 recesses of epidermis ; as these fall into decay, they become 

 gradually more and more liquid, being finally changed, in 

 the sudoriferous glands, into an extremely thin liquid. The 

 desquamation of the epidermis, in the foetus, is neither dry 

 nor corneous; it is distinguished by its fatty degeneration 

 (vernix caseosa), similar to the smegma prseputii ; this fatty 

 degeneration continues after birth in certain parts, especially 

 those which are formed last, such as the top of the head, and 

 especially about the median line and the great fontanel, the 

 skin of which appears at birth to be not yet fully matured. 



