296 



HISTOGENESIS 



central cavity of the anlage, the cells of which degenerate. When the 

 hair projects above the surface or the epidermis it breaks and carries with 

 it the epitrichial layer. The mesenchymal tissue which surrounds the hair 

 follicle in the neighborhood of the epithelial bed gives rise to the smooth 

 fibers of the arrector pili muscle. Pigment granules develop in the basal 

 cells of the hair and give it its characteristic color. 



Inner hair sheath 



Outer hair sheath - 

 Mesenchymal sheath 



Epidermis 



Arrector pili muscle fibers 

 Sebaceous gland 



Epithelial bed 

 Root of hair 



Hair bulb 

 Hair papilla 



FIG. 301. Longitudinal section through a developing hair from a five and one-half months' 

 human fetus (after Stohr). X 220. 



The first generation of 'lanugo' hairs are short-lived, all except those 

 covering the face being cast off soon after birth. The coarser, replacing 

 hairs develop, at least in part, from new follicles. Thereafter, hair is shed 

 periodically throughout life. 



Anomalies. Hypertrichosis refers to excessive hairiness which may be general or 

 local, as in the exhibited 'hairy monsters.' In the rare hypotrichosis, the congenital ab- 

 sence of hair is usually associated with defective teeth and nails. 



SWEAT GLANDS 



The sudoriparous, or sweat glands begin to develop in the fourth 

 month from the epidermis of the finger tips, the palms of the hands, and 

 the soles of the feet. They are formed as solid downgrowths from the 

 epidermis, but differ from hair anlages in having no mesenchymal papillae 

 at their bases. During the sixth month the tubular anlages of the gland 

 begin to coil, and, in the seventh month, their lumina appear. The inner 

 layer of cells forms the gland cells, while the outer cells become trans- 

 formed into smooth muscle fibers which here arise from the ectoderm. In 

 the axillary region, sweat glands occur which are large and branched. 



