Mammalia: Skin, Muscles, Skeleton 583 



ill reptiles (Fig. 454). Increased bulk of the muscles provides the 

 greater power needed. The weight of the leg muscles is a much greater 

 proportion of the animal's total weight in mammals than in reptiles. 

 Increase in number of muscles and modifications in their attachments 

 to skeletal parts give the animal better control and greater variety of 

 leg movements. The most bulky locomotor muscles are those massed 

 at the base of the limb (particularly the pelvic) and having their 

 proximal attachments on the dorsal region of the girdle and the neigh- 

 boring region of the vertebral column (Fig. 454, bottom). Consequently, 

 the dorsal portions of the girdles are more strongly developed than the 

 ventral parts. In the pectoral girdle the scapula is the all-important 

 bone, the ventral parts — coracoid, procoracoid, and clavicle — being, 

 in most mammals, more or less reduced or even completely absent 

 except that a rudimentary coracoid commonly fuses to the glenoid 

 region of the scapula to become the coracoid process of the scapula 

 (Fig. 125B). The clavicle is lacking in some marsupials, in ungulates, 

 cetaceans, and sea cows, and in some carnivores and a few rodents. 



Conspicuous superficial dorsal muscles of the pectoral appendage 

 are the trapezius and latissimus dorsi, opposed in their action by 

 ventral muscles which constitute the pectoralis group (Figs. 454, 455, 

 456). 



The relation of the leg to the ground while walking or running varies 

 (Fig. 450). In some mammals the entire length of maims and pes is 

 applied to the ground, the upward inclination of the leg beginning at 

 the region of wrist and heel respectively. This plantigrade condition 

 is illustrated by the bear and man. In most mammals only the ventral 

 surfaces of the digits are applied to the ground, the metacarpal and 

 metatarsal regions inclining more or less sharply upward so that wrist 

 and heel are clear of the ground — the digitigrade condition, seen in 

 dogs and cats. Minimum contact with the ground occurs in mammals 

 which walk on the distal tips of a much-reduced number of digits, 

 the distal phalanx of each digit being encased in a blunt, horny hoof. 

 The extreme of this unguligrade condition is seen in cattle and the 

 horse. In cattle the distal tips of only two digits of each leg touch the 

 ground; in the horse, only one. 



TRUNK 



Consequent upon the separation of the coelom into major anterior 

 and posterior divisions by the transverse diaphragm, the thoracic 

 body-wall and the abdominal body-wall become anatomically very 

 different. The differences are necessarily related to the functions and 

 mechanical requirements of the organs contained in the two cavities. 

 The heart and lungs must be protected from external impacts and 

 pressures. The lungs, incapable of filling themselves with air, are filled 



