Mammalia: Skin, Muscles, Skeleton 581 



reptiles from which the mammals have apparently been derived still 

 retained some integumentary glands inherited from amphibian 

 ancestors. 



In mammals integumentary sensory organs are present in 

 greater number and diversity of types than in reptilian skin. A dry, 

 scaly skin such as that of reptiles is not well adapted to being highly 

 sensitive. 



The "roots" of hairs, situated in elongated follicles (Fig. 443), and 

 the long tubular sweat-glands are both of epidermal origin, but they 

 burrow deeply into underlying tissue. Accordingly, the mammalian 

 dermis is usually relatively thicker than reptilian dermis. Also in 

 contrast to the reptilian dermis, whose connective-tissue fibers are 

 more or less definitely arranged in bundles and layers, the dermal 

 fibers in mammals are not arranged in parallel systems but are intri- 

 cately interwoven or "felted." 



The layer of subcutaneous connective tissue is more promi- 

 nently developed in mammals. It is highly elastic, admitting of move- 

 ment of skin in relation to the body-muscle. Such movements are 

 effected by thin sheets of subcutaneous muscle situated in the 

 subcutaneous layer but derived from the adjacent body-muscle. 

 These subcutaneous muscles are especially well developed in the 

 anterior region of the animal — e.g., the facial "muscles of expression" 

 in man (Fig. 96). 



Motor Mechanism — Muscles and Skeleton 



In general, the mammal excels the reptile in speed and agility and 

 is capable of a greater diversity of activities. These improvements are 

 most directly reflected in the anatomy of the motor mechanism and 

 the respiratory system. The changes in the proportions of the body 

 and in the arrangement of the coelomic spaces are consequences inci- 

 dental to the changes in the mechanisms of locomotion and breathing. 

 The anatomy of the skeleton is intelligible only in connection with the 

 organs which it serves to protect and, of broader importance, the 

 muscles whose strains are sustained by the skeletal parts. Therefore, 

 in the following description, muscles and skeleton will, so far as 

 possible, be considered together, beginning with the appendages as 

 the immediate locomotor organs and proceeding then to regions of 

 the body — trunk, neck, tail. 



APPENDAGES 



In connection with the greater length and altered positions of the 

 legs, the locomotor muscles are more elaborately differentiated than 



