INTEGUMENT 



33 



into which it wanders from the derm, which may also contain 

 pigment. 



In the derm, as may be seen by a glance at Fig. 25, an outer 

 papillary and an inner reticular portion may be distinguished. 

 The papillae of the former are accurately adapted to the over-lying 

 epiderm : some of them contain blood- and lymph-capillaries, and 

 others, nerves with tactile corpuscles. The latter, on the other hand, 

 becomes lost without any 

 sharp boundary line in the 

 subdermal connective tissue 

 and in the more or less 

 well-developed fatty layer 

 (panniculus adiposus). The 

 panniculus may be very 

 largely developed in aquatic 

 Mammals e.g. in the Ce- 

 tacea, in which it serves to 

 preserve the heat of the 

 body, and at the same time 

 to reduce the specific gravity 

 of the animal. 



As in Birds, the con- 

 nective tissue fibres of the 

 derm are irregularly felted. 

 Most of the smooth muscle- 

 fibres are inserted into the 

 hair-follicles (arrectores pilo- 

 rum, Fig. 23), but may 

 occur independently of hairs, 

 e.g., in the scrotum and 

 teats. 



In the great abundance 

 of integumentary glands, 



Mammals differ greatly from Reptiles and Birds, and more nearly 

 resemble Amphibians. They may be present in all parts of the 

 skin, and differ greatly with regard to the consistency, composition, 

 colour, and odour of their secretions. Those which serve for the ex- 

 cretion of products of destructive metabolism in general, and for the 

 formation of odoriferous substances, are either tubular or alveolar 

 in structure. The former, which were probably derived from those 

 of ancestral Amphibians, possess a muscular investment, have 

 mostly the form of the characteristically coiled sweat-glands, and 

 are rarely entirely wanting (e.g. Cetacea) : the latter, which are a 

 new acquisition and are known as sebaceous glands, appear to be 

 not only functionally, but also ontogenetically and phylogenetic- 

 ally closely connected with the hairs (Figs. 23 and 25). 

 Various modifications of both kinds are met with, and they 

 are often arranged in groups. Thus the nasolabial glands of 



D 



FIG. 25. SECTION THROUGH THE HUMAN 

 SKIN. 



1 o, derm ; F, subcutaneous fat ; GP, vascular 



papillae ; Sc, stratum corneum ; SD, sweat- 

 glands, with their ducts (SD l )',SM, stratum 

 Malpighii. 



