314 EXCRETION 



stance that gives to the hair the variety in color observed in different 

 persons. In the lower part of the root the fibres are much shorter, and 

 at the bulb they are transformed into the soft rounded cells found in 

 this situation and covering the papilla. 



The epidermis of the hair is thin and is composed of flattened 

 quadrangular plates, overlying each other from below upward. These 

 scales, or plates, are without nuclei, and they exist in a single layer over 

 the shaft of the hair and the upper part of its root ; but in the lower 

 part of the root, the cells are thicker, softer, frequently are nucleated, 

 and they exist in two layers. 



The medulla is found in the short stiff hairs, and it is often quite 

 distinct in the long white hairs of the head. It occupies one-fourth to 

 one-third of the diameter of the hair. The medulla can be traced from 

 just above the bulb to near the pointed extremity of the hair. It is 

 composed of small, rounded, nucleated cells, which often contain 

 dark granules of pigmentary matter. Mixed with these cells are air- 

 globules ; and frequently the cells are interrupted for a short distance 

 and the space is filled with air. The medulla likewise contains a gluti- 

 nous substance between the cells and surrounding the air-globules (see 

 Plate VII, Fig. I, for transverse sections of hairs). 



Growth of the Hairs. Although not provided with blood and 

 devoid of sensibility, the hairs are connected with vascular parts and 

 are nourished by imbibition from the papillae. Each hair is first de- 

 veloped in a closed sac, and at about the sixth month of intra-uterine 

 life, its pointed extremity perforates the epidermis. These first-formed 

 hairs are afterward shed, like the milk-teeth, being pushed out by new 

 hairs from below, which latter arise from a second and more deeply 

 seated papilla. The shedding of the hairs usually takes place between 

 the second and the eighth month after birth. 



The differences in the color of the hair depend on differences in the 

 quantity and the tint of the pigmentary matter ; and in old age the hair 

 becomes white or gray from a blanching of the cortex and medulla. 



Sudden Blanching of the Hair. There are a few instances on 

 record in which sudden blanching of the hair has been observed and 

 the causes of this remarkable change fully investigated. One of these 

 cases has been reported by Landois. In this instance the blanching 

 of the hair occurred in a hospital in a single night, while the patient, 

 who -had an acute attack of delirium tremens, was under the daily obser- 

 vation of the visiting physician. The microscopical examinations by 

 Landois and others leave no doubt as to the cause of the white color of 

 the hair in cases of sudden blanching ; and the fact of the occurrence 

 of this phenomenon can no longer be called in question. All are agreed 



