VERTEBRATA : MAMMALIA. 643 



prsesternum is the "manubrium sterni" of human anatomy, 

 and is the portion of the sternum which lies in front of the 

 attachment of the second pair of ribs. All the other ribs are 

 connected with the mesosternum. The xiphisternum is the 

 "xiphoid cartilage" of human anatomy, and it commonly 

 remains throughout life more or less unossified. In the Mono- 

 tremes there is a T-shaped bone above or in front of the 

 praesternum, but this is to be regarded as belonging to the 

 shoulder-girdle, and as representing the "episternum" or 

 " interclavicle " of the Reptiles. 



The normal number of limbs in the Mammalia is four, two 

 anterior and two posterior ; and hence they are often spoken 

 of as "quadrupeds," though all the limbs are not universally 

 present, and other animals have four limbs as well. The ante- 

 rior limbs are not known to be wanting in any Mammal, but 

 the posterior limbs are absent in the Cetacea and Sirenia. 



As regards the structure of the anterior limb, the chief points 

 to be noticed concern the means by which it is connected with 

 the trunk. The scapula or shoulder-blade is never absent, and 

 it is in the form of a broad flat bone (rarely long and narrow), 

 applied to the outer aspect of the ribs, and much more devel- 

 oped than in the Birds. The coracoid bone, which forms such 

 a marked feature in the scapular arch of Aves, is fused with the 

 scapula, and only articulates with the sternum in the Duck- 

 mole and Echidna (Monotremata). In all other Mammals the 

 coracoid forms merely a process of the scapula, and does not 

 reach the top of the breast-bone. The collar-bones or clav- 

 icles never unite in any Mammal to form a " furculum," as in 

 Birds ; but in the Monotremes they unite with an " interclav- 

 icle," placed in front of the sternum. The clavicles, in point 

 of fact, are not present in a well-developed form in any Mam- 

 mals except in those which use the anterior limbs in flight, in 

 digging, or in prehension. The Cetacea, the Hoofed Quadru- 

 peds ( Ungulatd) and some of the Edentata, have no clavicles. 

 Most of the Carnivora and some Rodents possess a clavicle, 

 but this is imperfect, and does not articulate with the top 

 of the sternum. The Insectivorous Mammals, many of the 

 Rodents, the Bats, and all the Quadrumana, have (with Man) 

 a perfect clavicle articulating with the anterior end of the 

 sternum. 



The humerus, or long bone of the upper arm (brachimn), is 

 never wanting, but is extremely short in the Whales, in which 

 the anterior limbs are converted into swimming-paddles. In 

 many Mammals, as in the Monkeys, and Felidce (constituting 

 the most typical group of the Carnivora], the median nerve and 



