2G 



COMPARATIVE ANATOMY 



A richer hairy covering (lanugo) is often met with in the embryonic 

 condition as, for instance, in the human foetus than occurs later ; and this 

 fact, together with the occasional appearance of abnormally hairy individuals, 

 indicates that at one time Man was distinguished by a far more abundant 

 clothing of hair than at the present day. 



Other epidermic structures, formed as thickenings of the horny 

 layer, also play a very important part in Mammals ; such are 

 daws, nails, bristles, and spines (Hedgehog, Porcupine) ; the so- 

 called whalebone (baleen) of the Mystaceti ; the horn-sheaths in 

 Ruminants ; the nasal horns of the Rhinoceros ; the scales in Manis 

 and on the tail of the Beaver and other Mammals ; the palatal 

 plates of Sirenia ; and the ischial callosities of certain Apes. 



When pigment is present, as, for instance, on the snout in many 

 Mammals and on the external genitals (labia rnajora and scrotum) 

 and the teats in the human subject, it is always situated in cells 

 of the Malpighian layer. 



The outer layer of the dermis, as may be seen by a glance at 

 Fig. 16, B, may be divided into an outer papillary and an inner 



reticular portion. The pa- 

 pillae of the former are ac- 

 curately adapted to the 

 over-lying epidermis : some 

 of them contain blood- and 

 lymph-capillaries, and others, 

 nerves with tactile cor- 

 puscles. The latter, on the 

 other hand, becomes lost 

 without any sharp boundary 

 line in the sub-dermal con- 

 nective-tissue and in the 

 more or less strongly-de- 

 veloped fatty layer (panni- 

 culus adiposus). The pads 

 (tori) on the soles of the feet 

 of most Mammals are due to 

 large dermal papilla. 



In addition to numer- 

 ous elastic fibres, smooth 

 muscle elements are distri- 

 buted throughout the der- 



16, B. SECTION THROUGH THE HUMAN 

 SKIN. 



.Sc, stratum corneum; SM, stratum Malpighii ; 

 Co, dermis ; F, F, subcutaneous fat ; NP, 

 sensory papillse ; GP, vascular papillae ; N 

 and G, nerves and vessels of the dermis ; 

 SD, SD, sweat-glands, with their ducts 

 (SD 1 , SD 1 ) ; H, hair with sebaceous 

 glands (D). 



mis ; they are particularly 

 abundant in the scrotum 

 (dartos) and in the teats, 

 and are present in connec- 

 tion with the hair-sacs 



(arrectores pili) : the power of erecting the hair possessed by many 

 Mammals is due to these (Fig. 16, A). A bony dermal skeleton 

 is found only in the Armadillo amongst existing Mammals 

 (comp. p. 34). 



