258 COMPARATIVE ZOOLOGY OF CHORDATES 



by the contraction of these (smooth) muscles in mammals 

 that hair is made to " stand on end," and the puckering of 

 the skin round the hair-follicles gives rise to the condition 

 known as " chicken-skin. " 



In addition to these dermal muscles, there are in the higher 

 forms, and especially in the mammals, sets of muscles beneath 

 the skin and which move the skin as a whole. The panniculus 

 carnosus muscles are in the region of the trunk and they serve 

 to shake the skin. (They are of somatic origin.) In the head 

 and neck regions the platysma muscles (of visceral origin) 

 move certain parts of the skin such as the lips, eyebrows, and 

 ears. In man, these are the muscles of expression. The 

 smooth dermal muscles are innervated by sympathetic fibres, 

 the panniculus carnosus by ventral nerve-roots, and the 

 platysma by the facial nerve. 



Just as the cells of the epidermis seem to be prone to the 

 production of horn and horn-like structures, so the cells of the 

 dermis seem to run to the formation of bone and dentine. 

 Dentine is the substance of which denticles and teeth are 

 formed, under the epidermal cap of enamel. The bone pro- 

 duced in the dermis takes the form of dermal or membrane- 

 bone, bony scales, or fin-rays (lepidotrichia). In Selachii the 

 dermis forms dentine but no bone. 



Dermal bones are widely distributed over the body in forms 

 above the Selachii. They play an important part in the 

 formation of the skull, and of the pectoral girdle. In some 

 animals, the body may be entirely covered by an armour of 

 bony plates, as in the Labyrinthodonts, or the armadillos. 

 These bony plates are osteoscutes, and remnants of them are 

 to be found in the carapace and the ventral shield (or plastron) 

 of the tortoise, and in the so-called abdominal ribs or gastralia 

 of Sphenodon, crocodile, Plesiosaurs, Ichthyosaurs, Pterosaurs, 

 and Archaeopteryx (see Fig. 160). Osteoscutes are also present 

 in Gymnophiona, lizards, and crocodiles. 



In the fish, the dermal bones come into relation with the 

 overlying denticles, forming complex scales. In the 

 Osteolepidoti and primitive (extinct) Dipnoi, the denticles 

 have fused together forming a layer of " cosmin," and this 



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