672 COMPARATIVE ANATOMY 



claws as well as scales, though the latter are not so conspicuous in the 

 higher forms. 



The corium is quite thick and is composed of irregular fibers inter- 

 laced with muscles, blood vessels, etc. Its outer surface often forms 

 papillae or ridges; especially on the palms and soles. These ridges carry 

 the epidermis with them. Several strata may usually be recognized 

 under the epidermis, namely : a thick Malpighian layer at the base, then a 

 thin stratum lucidum in which distinct cells cannot be recognized, and 

 the stratum corneum on the outside. One or more other layers may be 

 present. A cell must pass through all of these layers "before it is worn 

 from the surface of the skin. 



HAIR 



It is important that the histological structure of a hair (Figs. 393, 

 394) be compared with that of a feather already described. 



Scales are found in many orders, usually best developed on the tail 

 and feet. They are rounded, quadrangular, or hexagonal, the square 

 scales being arranged in rings around the part, the others in groups of 

 five known as quincunx. These latter are closely similar to the scales 

 of reptiles. It seems, from recent investigations, that there is a close 

 relation between scales and hair, since in mammals with scales, hairs are 

 usually arranged in groups of three or five behind each scale ; and in 

 those without scales, the hairs are also grouped in the same manner. In 

 the early embryo, the hairs are arranged in longitudinal rows so that 

 grouping seems to come later. 



GLANDS 



These are of various kinds and types. The structural shapes and 

 forms into which they may be grouped have already been studied in the 

 frog and should be recalled, but we must also think of five divisions or 

 groups, classified not according to structure, but according to function. 

 Thus we have the following grouping : 



(1) Sweat (tubular in shape extending from the Malpighian layer 

 down through the corium where they are coiled). 



Schematic arrangement of varying types of mammary glands. 1, Echidna, 

 primitive type; 2, Halmaturus (a genus of Kangaroo) forming pouch in lacta- 

 tion ; 3, Didelphys, forming of nipple before lactation ; 4, Same during lactation 

 (quite like man) ; 5, Mammilary pouch in cow embryo; 6, in adult cow. (After 

 Max Weber.) 



