THE HAIRS 



1291 



In colour the hairs may be either blonde, brown, black, red, or some gradation 

 of these colours. The colour varies with the race, and also with the individual, 

 and according to age. It is due to pigment in the cells of the hair but is also 

 influenced by the amount of air between the cells. 



Greying and whitening of the hair is due not only to a decrease of pigment but also to an 

 increase in the amount of air between the cells. Sudden blanching of the hair is thought to be 

 due almost entirely to an increase in the quantity of this contained air. Whitening of the hair is 

 physiological in old age and not infrequent in younger persons. This may be an inherited pecul- 

 iarity or may follow mental overwork, nervous shock, or prolonged disease. Local blanching is 

 also seen as the result of disease. 



The hair may be straight, waved, curled, or frizzled in varying degree. Here also there is 

 not only an individual but also a racial variation, as instanced in the curled or crinkled hair of 

 the African negro and the straight hair of the American Indian. The curliness is caused by the 

 form and the manner of implantation in the skin. Straight hairs are round or oval in transection 



Fig. 1050.- 



-LONGITUDINAL SECTION OF A GROWING HaIR OF THE HeAD. 



Toldt's Atlas.) 



( X30.) (From 



Scapuspili (shafts. 



■ Substantia corticalis 



Substantia medullaris 



Collum folliculi pili" — 



. I 



Sebaceous gland 



Epidermal 

 coat of 

 follicle 



Inner root sheath - 



Outer root sheath "" r 



Radix pili (root)- 



,/ \ Arrector pili muscle 



Dermal 

 coat of 

 folUcle 



Outer fibrous layer ■ — 

 Inner fibrous layer ■ — ' 

 Hyaline layer .^ * 



"I i ''VI 



Bulbus pil 



Fundus folliculi 



Papilla pili 



and curled hairs are more flattened. The root of curled hair has been observed in certain 

 instances, as in the negro, to have a curved course in the skin which may account in a measure 

 for its curliness. 



The hairs are arranged singly or in groups of from two to five and, except those of the eye- 

 lashes, are implanted at oblique angles to the surface of the skin. The directions in which the 

 hairs point are constant throughout life for the same individual. They are arranged in tracts 

 in which the hairs diverge from a centre in whorls, the vortices pilorum. 



These vortices are found constantly in certain definite regions and apportion the whole hairy 

 surface. The centres of vortices are found at the vertex (sometimes double) upon the face, 

 around the external auditory meatus, in the axilla, in the inguinal region, and sometimes on the 

 lateral sm-face of the body. These are all paired except as a rule the first. Where adjoining 

 vortices come together the hairs are arranged in lines along which they all point in nearly the 

 same direction, only slightly diverging, forming the hair streams, fiumina pilorum. In other 

 Unes and places the hairs point in converging directions such as at the umbilicus and over the 

 tip of the coccyx. 



