INTRODUCTION. XV11 



The thorax is inclosed by ribs, some of which, called the true 

 ribs, are attached to the sternum by a cartilage, the representative 

 of the sternal ribs of other Vertebrata. The posterior ribs are called 

 the false or floating ribs. The sternum is composed of several pieces, 

 and gives attachment to the clavicles when present. The ribs are 

 capable of considerable motion of elevation and depression, aiding the 

 diaphragm in respiration. 



The anterior extremity is fixed to a broad scapula, generally 

 only connected by muscles to the trunk. A clavicle is present in 

 those Mammals that use the arm as an instrument of prehension or 

 flight, and the coracoid bone, so conspicuous in other Vertebrata, 

 is reduced to an appendage. The rest of the limb is composed of 

 the humerus, two bones of the forearm, the radius and ulna, 

 the carpus, metacarpus and phalanges. These last vary much in 

 number. 



The posterior extremity consists of the pelvis, comprising the iliac, 

 ischial and pubic bones, the femur or thigh bone articulated with the 

 pelvis, the tibia and fibula, tarsus, metatarsus, and phalanges. 

 Modifications of some of these bones, and reductions of many of 

 them, occur among various Mammals, and will be pointed out in the 

 proper place. 



Considered generally, the muscular system of Mammals varies 

 little from that of Man. The cutaneous muscles are much developed 

 in some, especially in the Porcupine and Hedgehog, by means of which 

 the spines can be suddenly raised. The same is noticeable to a less 

 degree in Dogs and Cats, when they, as it is popularly called, (r get 

 their backs up." 



Among Mammals the demand for food is less constant, and the 

 digestive process less rapidly accomplished than in Birds. There 

 is a considerable amount of uniformity in the structure of their 

 digestive organs. The tongue varies much, and is free in most, 

 being only fixed in the Whales. In some it is capable of great 

 extension, as in the Ant-eaters, and is used to procure food, as it is 

 also, to a certain extent, by some Ruminants. The surface of 

 the tongue is usually covered with papillae, which in some of 

 the Carnivora, the Cat tribe notably, are developed into sharp, 



b 



