450 DISEASES OF TUB SKIN. 



the case among older persons. As in premature baldness, hereditary 

 tendency seems to be the main cause of premature grayness of the 

 hair, although, perhaps, care and sorrow may also play some part in 

 producing the change. It is a curious fact that a hair, which is only 

 gray at its tip, if pulled out, does not become gray throughout, but re- 

 mains permanently in the same condition. From this circumstance it 

 would seem that the matrix exerts some vital influence in rendering 

 the hair gray. This fact, moreover, renders those stories, which have 

 been handed down to us, far less incredible, although their truth has 

 been much doubted, of the hairs becoming gray in a few days from 

 the effect of intense mental emotion ; all the more so, since, according 

 to Pfaff) the grayness does not depend upon a disappearance of the 

 pigment from the hair, but upon a thickening of its cortical layer. In 

 a case reported by Landois, sudden grayness of the hair took place in 

 consequence of an increased development of air in the hair. The pig- 

 ment does not disappear from the medullary substance of the hair un- 

 til in advanced old age. 



III.HYPERJEMIA AND ANAEMIA OF THE SKIK 



THE quantity of blood contained by the skin varies more than that 

 contained by any other organ, because the skin is far more exposed 

 than other parts of the body to influences capable of modifying the 

 circulation. In our first volume we have already given a detailed ac- 

 count of those abnormities of the cutaneous circulation which arise 

 from disease of the heart, including both the overloading of the ar- 

 teries and arterial capillaries from increased cardiac action, and the 

 engorgement of the veins and venous capillaries (cyanosis), which re- 

 sults from lack of power in the heart. 



Active hyperaemia of the skin (fluxion) occurs upon exposure of 

 the skin to great heat, especially to heat and moisture combined. It 

 also may result from the application upon it of irritating substances, 

 such as cantharides, mustard, or spurge-laurel, or from mechanical in- 

 jury. In all such cases, as has repeatedly been mentioned, the first 

 effect of the irritant seems to be a relaxation of the cutaneous tissue ; 

 and a dilatation of the capillaries seems to be consequent upon the re- 

 duced power of resistance of the ti&sues which surround them. If the 

 hyperaemia be intense enough to produce a redness of the skin, visible 

 through the epidermis which covers it, it is called erythema, and is fur- 

 ther distinguished as erythema caloricum, solar e, venenale, according 

 as the redness proceeds from heat, the sun's rays, or the action of mus- 

 -ard, cantharides, or spurge, and the like. With equal reas m, redness 



