. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative. COIMPARATIVE 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 ai'e^— daus, ncnh, bristles, and spines (Hedg
. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative. COIMPARATIVE 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 ai'e^— daus, ncnh, bristles, and spines (Hedgehog, Porcupine); the so- called whalebone (baleen) of the Mystaceti; the horn-sJieaths in Ruminants; the nasal horns of the Rhinoceros; the scales in Manis and on the tail of the Beaver and other Mammals; the palatal 2)Iiiies of Sirenia ; and the ischial callosities of certain Apes. When piigmcnt is present, as, for instance, on the snout in many Mammals and on the external genitals (labia majora 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- pillte 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 Sj\f 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 papillae. In addition to numer- ous elastic fibres, smooth muscle elements are distri- buted throughout the der- mis ; they are particularly abundant in the scrotum {dartos) and in the teats, and are present in connec- tion with the hair-sacs {arn-ctores pili): the power of erectin
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectanatomycomparative
