34 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



new cells thus formed are forced outward, forming the hair. While 

 this is going on the ingrowth splits around the hair, forming the follicle, 

 while another ingrowth of the Malpighian layer forms the sebaceous 

 gland which oils the hair. 



A section through a hair and its follicle gives the following layers (fig. 26). 

 Around all is the connective-tissue envelope, formed from the corium; next inside 

 is the outer root sheath formed of the Malpighian layer and extending to the cavity 

 of the follicle. Around the root of the hair is the inner root sheath, two cells in 

 thickness, the layers being known as Henle's and Huxley's layers. These do not 

 extend outside the follicle. In the hair itself there is a cortical layer surrounding 

 the central medulla, the hair not being hollow. 



FIG. 26. Diagram of structure of hair, b, blood-vessels; ct, cuticle of hair; ex, cortex g, 

 gland; h, hair; he, Henle's layer; hf, hair follicle; hoc, Huxley's layer; m, medulla; p, papilla; 

 sg, stratum germinativum of epidermis. 



Hair differs greatly in size, the spines of the porcupines forming one extreme, the 

 prenatal hair (lanugo) of man the other. Hair is shed at intervals. The old hair 

 ceases to grow, separates from its base, and later is pushed out when the root begins 

 again to proliferate. There are smooth muscle fibres connected with the roots of 

 the hairs, their function being to raise the hair from its usual inclined position under 

 influence of the sympathetic system. There are also usually nerves distributed 

 to the base of the hairs, making them to some extent sense organs, a condition 

 which reaches its greatest development in the facial hairs (vibrissae) of carnivores 

 and the hairs on the wings of bats. 



Scales occur in several orders, being usually best developed on the 

 tail and feet. They may be rounded, quadrangular or hexagonal, the 

 square scales being arranged in rings around the part, the others in 

 quincunx. These are closely similar to the cuticular scales of reptiles 

 (p. 26). Recent investigations tend to show that there is a close rela- 



