256 



THE TISSUES. 



rectangular or quadrangular, and rarely rounded or fusiform, 

 to TsVir f an i ncn i n diameter, containing dark, fat-like granules, 

 and a clear nucleus, ^^ to goVu of an i ncn i Q diameter. The 

 granules are, however, not fat nor pigment, but merely air-vesicles. 



Fig. 161. 



162. 



Fig. 161. Plates of cuticle of hair, &c. a, b. Transverse sections with and without pigment in the 

 centre, c. A longitudinal section showing imbrication, and the pigment in the fibrous portion, d. 

 Cortical plates showing edges, d'. Margin of same, showing their imbrication. (Magn. 150 diameters.) 



Fig. 162. Hair-bulb, root-sheath, its epidermis, &c. a. Medulla containing air-cavities and indis- 

 tinct cells. 6. Fibrous substance, c, d. Inner and outer layers of cuticle. e,f. Inner and outer 

 layers of internal root-sheath, g. External root-sheath, h. Basement membrane, i. Transverse 

 fibre-stratum. Tc. Longitudinal fibre-stratum. I. Papilla, m. Lowest cells of hair-bulb continuous 

 with those of external root-sheath, n. Perpendicularly placed nucleated cells, becoming non-nucle- 

 ated nearp, and continuous with the inner layer of the cuticle, o. Small perpendicularly ai-ranged 

 nucleated cells passing into the outer layer of the cuticle, p. Lowest portion of the inner root-sheath. 

 q. .Union of cuticle with fibrous substance, r. Commencement of the medulla in colorless cells, s. 

 Part where the cells of the bulb begin to lengthen, to form the fusiform plates and cells of the shaft. 



They vary, according to the hairs, from go&^o- to sdw f an 

 in diameter, and occupy the medullary cells in great amount, exist- 

 ing both in white and in dark hairs. In the latter the air appears 

 of a brown-red or brown tinge, from being seen through the colored 

 fibrous substance ; in white hairs it is of a silver white. It appears 

 certain that the air may pass from one air- vesicle to another in the 

 hair. Just above the bulb, and sometimes also in spots in the shaft, 

 there are some of these air- vesicles, and therefore a paleness results. 

 In some hairs, especially the red, there is often no definite line of 

 demarcation between the fibrous portion and the medulla. 



The medulla usually constitutes from one-fifth to one-third of the 



