DEVELOPMENT OF THE SKIN 



583 



Fig. 274. Diagrams of nails, claws and hoofs. (A redrawn and modified from Pinkus, 

 Chap. 10. The Development of the Integument, from Keibel and Mall, 1910. Vol. I, 

 Lippincott, Phila.) (A) Longitudinal section of index finger of human fetus of 8.5 

 cm. (B) Longitudinal section of human finger, showing relationships of fully developed 

 nail plate. (C) Claw of the cat. (D) Cloven hoof of the pig. (E) Developing hoof 

 of pig. (F) Uncleft hoof of horse, lateral view. (G) Uncleft hoof of horse, ventral view. 



cells, the root contains the cells in a viable condition before transformation 

 into the cornified state. The root of the hair consists of the hair papilla, com- 

 posed of dermal mesenchymal cells, blood vessels, nerve fibers, and a cup- 

 shaped epithelial matrix which overlies the papilla (fig. 273 A). The hair 

 shaft and the internal root sheath are derived from the modification of the 

 cells of the hair matrix. The internal root sheath is composed of the inner 

 sheath cuticle, together with Huxley's and Henle's layers (fig. 273B). The 

 internal sheath disappears in the upper regions of the follicle near the entrance 

 of the sebaceous gland. External to the internal root sheath is the external 

 root sheath. The latter represents the wall of the epithelial follicle and is the 

 downward continuation of the epidermal layer of the skin around the root of 

 the hair. The external root sheath thus forms a pocket-like structure, extend- 

 ing from the distal margin of the hair matrix to the epidermis of the surface 

 skin. A sheath of dermal cells and fibers lies around the external root sheath 

 and acts as the skeletal support of the hair. 



During development, hair first appears in the region of the eyebrows and 

 around the mouth. Later it develops over the surface of the body in a regular 



