THE HAIE 219 



of the derma, respectively. The dermal root sheath is, however, 

 devoid of papillae. 



The glassy membrane is a peculiarly thick homogeneous mem- 

 brane which is chiefly mesoblastic in origin, but whose innermost 

 portion (Kolliker,* Stohr f) is formed as an exoplasmic product of 

 the adjacent epithelium. This membrane is highly refractive and 

 contains very few connective tissue cells or fibres. 



The circular fibres of the dermal root sheath contain inter- 

 lacing bundles of connective tissue fibres, which are mostly dis- 

 posed in a ring-like manner. Elastic fibres are absent. Within 

 this layer is a dense anastomosing plexus of capillary blood vessels, 

 together with a rich subepithelial plexus of non-medullated nerve 

 fibres. 



The longitudinal fibres of the connective tissue root sheath 

 also form interlacing fibre bundles, most of which are somewhat 

 obliquely disposed. The bundles are coarser than those of the 

 preceding layer and contain "a few elastic fibres. This portion 

 of the root sheath contains many small blood vessels and nerves 

 which supply the plexuses of the circular layer. 



Atypical Portions of the Hair Follicle. As already indicated, 

 the hair follicle presents some structural differences at various 

 levels. The typical arrangement is found only in the mid-portion 

 of the follicle. 



In its superficial portion the hair lies free in the follicular 

 lumen, the interval between it and the inner root sheath being 

 only partially occupied by the fatty secretion of the sebaceous 

 gland which enters the lumen of the follicle at the deeper por- 

 tion of its middle portion. At this level also, the root sheaths of 

 the hair offer a gradual transition from their typical structure to 

 that of the dermal and epidermal layers with which they are con- 

 tinuous. 



The hair bulb likewise differs prominently from the typical 

 structure of the hair root. In this portion the Malpighian layers 

 are very highly developed at the expense of the horny layers. It 

 is, therefore, in this portion that growth is most active. The cells 

 of this region are often deeply pigmented. The increased size of 

 the Malpighian layer, moreover, produces a distinct bulging of the 

 follicle, which incloses the hair papilla and results in the peculiar 

 bulbous shape of the extremity of the hair follicle. 



* Handbuch. f Anat. Hefte, 1903. 



