402 MOEBID ANATOMY OF THE SKIN. 



abscesses thus formed, burst, and discharge their contents. 

 Tumours and cicatrices follow the same law as in leprosy and 

 syphilis. 



e. Atrophy, 



§ 347. The falling-oflP of the hair of the head in old age is 

 due to a total involution of its place of origin, i.e. of the hair- 

 sacs and the roots of the hair. The former are dilated and 

 shortened, the latter grow smaller, and either disappear or are 

 only capable of producing and nourishing a lanugo. 



Premature baldness is invariably connected with a dis- 

 ordered relation between the production of epidermic cells from 

 the hair-sac on the one hand, and from the hair-root on the 

 other. If cells are formed in excessive numbers by the lining of 

 the sac, the lateral pressure to which the hair is exposed in the 

 interior of the follicle, interferes with the nutrition of the shaft, 

 and causes it to wither and become spontaneously detached from 

 its root. This occurs in the '^defluvimn capillorum" which is 

 met with in the course of constitutional syphilis, or after acute 

 diseases, such as typhus abdominalis. 



The second and far rarer case is when the productive activity 

 of the root sinks below its normal standard. The hair-follicle is 

 in nowise altered, but the normal degree of lateral pressure 

 exerted upon the shaft by the narrowest part of the follicle (just 

 below the point at which the sebaceous glands open into it) 

 is too great to be overcome by the diminished upward strain 

 of the growing haii\ The latter is accordingly arrested at 

 this point ; its cells undergo a finely-granular metamorphosis, 

 preliminary to a solution of continuity which is effected by the 

 least traction upon the shaft from without (as in combing the 

 hair). The remaining stump is very soft ; it becomes swollen 

 and knotty, owing to the fact that the materials furnished by the 

 root, though inadequate for the construction of a normal hair, 

 yet form, in course of time, a shapeless corpuscular aggreo-ate of 

 considerable size. {Alopecia areata. Area (7ehi.) 



