TISSUES OF THE BODY. 



389 



it may project downwards into the subcutaneous cellular tissue. Its 

 form is in general cylindrical ; not seldom, however, it is narrowed at its 

 lower end. It consists, like the corium, of fibrous connective-tissue, in 

 which several layers may be observed. To this one or several bundles of 

 unstriped muscle are attached externally, the arectores pili of Eylandt. 

 The outermost layer of the follicle, which may be extremely thin when 

 the surrounding tissue is densely interwoven, is seen to be made up of 

 longitudinally arranged connective-tissue bundles with fusiform nuclei 

 lying in the same direction. The thick- 

 ness of this layer usually ranges be- 

 tween 0-0036 and 0-0070 mm. In it a 

 complicated network of capillaries is to 

 be seen, and nerves have been in some 

 instances also observed. 



The middle layer of the hair follicle 

 is in general somewhat thicker, mea- 

 suring from 0-0149 to 0'0233 mm. It 

 consists of undeveloped connective- 

 tissue, whose fibres have a transverse 

 direction, with several layers of elon- 

 gated nuclei, whose appearance brings 

 to our recollection that of the well- 

 known nuclei of involuntary muscle 

 fibres (Koelliker), although no such 

 elements can be clearly demonstrated 

 here. A capillary network is also to be 

 seen here, whose meshes have a direction 

 chiefly transverse. This middle layer 

 commences at the bottom of the fol- 

 licle, but terminates above on arriving 

 at about the neighbourhood of the seba- 

 ceous glands. The human hair follicle, 

 further, is surrounded by lymphatic 

 vessels. 



Finally, the whole induplication is 

 enclosed in a transparent structureless 

 membrane (fig. 389, b- } fig. 390, g) 

 finely striated internally, which may 

 be looked upon as a modified limiting 

 membrane or hyaline coat. Like many 

 structures of the same nature, it mani- 

 fests great power of resisting the action 

 of acids and alkalies. Between this 

 layer and the middle, in the large tactile 

 hairs of some mammals, there is situated, according to Leydig and Odenius, 

 a highlyTdeveloped cavernous vascular plexus, which terminates above in 

 a circular venous sinus (Dietl and School). 



According to the more recent investigations of Wei'theim, however, the 

 hair follicle is not, as has been up to the present generally supposed, 

 rounded off at its termination in the manner represented in our fig. 389. 

 It is continued down through its external and middle layers into a cord 

 of connective-tissue, which becomes at first enlarged " like a cup," and 

 then narrowed into a kind of " stalk." Preserving the direction of the 



Fig. 389. Human hair and follicle, o, 

 fibrous follicle; 6, transparent internal 

 layer of the latter; c, the external, and rf, 

 the internal root-sheath ; e, transition of 

 the external sheath into the hair bulb ; /, 

 nair-cuticle, seen at /* in the form of 

 transverse fibres ; g, the lower portion of 

 this structure; h, cells of the hair-bulb; 

 f, hair papilla; *, cells of the medullary 

 part ; /, cortical portion ; m, medulla con- 

 taining air; n, transverse section of the 

 latter; o, cortex. 



