MEMBERS OF MAMMALIA. 107 



Sternum is generally narrow and flat ; but amongst the Bats, 

 where the lower muscles of the wing are required to have great 

 power, and must find a large surface for their insertion in this 

 bone, it often presents on the medial line an elevated crest, which 

 somewhat resembles the keel of the breast-bone in Birds. In 

 all the animals of this class, the thoracic cavity is separated 

 from the abdomen, by a complete division formed by the dia- 

 phragm. 



88. Members. The Members are four in number among all 

 the ordinary Mammalia ; but in the Whale, and other fish-like 

 Mammalia usually known under the name of Cetacea, there are 

 only two, the abdominal members being absent. As in Man, 

 these organs are always composed of a jointed lever, which is 

 divided into three principal parts; the arm or the thigh, the 

 fore-arm or the leg, and the hand or foot; but the mode of con- 

 formation of these different parts varies a little ; in accordance 

 with the uses to which they are destined. 



89. The frame- work to which the upper or thoracic extre- 

 mity is attached, is principally composed of a large flat bone, 

 which is supported on the ribs, and which gives an attachment to 

 the humerus; this is called the blade-bone, or Scapula (0, Fig. 51 ). 

 This bone is prolonged in a direction parallel to the vertebral 

 column, in proportion to the violence of the efforts, which the 

 animal is destined to make with the member attached to it ; and 

 by this extension, it affords greater space for the attachment of 

 the muscles, which draw the arm towards the body. In those 

 Mammals which employ their upper extremities for prehension, 

 or as organs of flight, and which require to draw them inwards 

 forcibly towards the chest, we find that the scapula is kept in its 

 proper position by means of the clavicle, which is united to it at 

 one extremity, whilst by the other it bears against the sternum 

 (Fig. 85) ; but in those species, in which movements of this 

 kind are but little or not at all required, the clavicle is completely 

 wanting, or only a rudiment of it is found. This is the case in 

 all the hoofed Quadrupeds, as well as in many others. In cer- 

 tain of the very remarkable Mammalia of New Holland, such as 

 the Ornithorhyncus, on the contrary, the bones of the shoulder 



