38 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



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 surrouding the central medulla, the hair not 

 being hollow. 



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 fibers connected with 

 the roots of the hairs, their function being to raise the hair from its usual in- 

 clined 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. 



Fig. 29. — 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; hx, Huxley's layer; m, 

 medulla; p, papilla; sg, stratum germinativum of epidermis. 



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 30). Recent investigations tend to show that there is a close 

 relation between scales and hairs, since in the mammals with scales 

 the hairs are usually arranged in groups of three or five behind each 

 scale (fig. 30) ; while in those without scales the hairs are frequently 

 grouped in the same manner. The illustration (fig. 31) is interest- 

 ing as showing the arrangement in man and the possible relation to 

 ancestral scales. The statement is also made that in the early 

 embryo the hairs are arranged in longitudinal rows and that the 

 grouping comes later. 



