652 MAMMALIA [CH. 



collar-bone or clavicle is attached; this bone extends inwards to 

 the sternum and is loosely connected with it. In some Mammals 

 the clavicle is absent. 



The general form of the pelvic girdle to which the hinder 

 limbs are attached is not very unlike that of Birds ; but there are 

 two important differences. First, the ilia or hip-bones are attached 

 only for a very short distance to the backbone ; and secondly, the 

 lower bones of the girdle, the pub is and ischium, meet their 

 fellows of the opposite side in front of the belly beneath the 

 anus, whereas in Birds they do not even approach each other in 

 this place, though in Rhea the ischia do meet dorsal to the anus. 



The leg of Mammals differs from that of Birds and Reptiles 

 in that the ankle-joint is situated between the bones of the 

 shank (the tibia and the fibula) and the small bones of the ankle, 

 instead of in the middle of these small bones (Fig. 322). The 

 heel-bone or calcaneum is one of the uppermost tier of ankle- 

 bones and corresponds to the bone called fibulare in the general 

 scheme of the pentadactyle limb. It is prolonged into the heel, 

 to which the great gastrocnemius muscle which forms the calf of 

 the leg and which raises the heel is attached. 



Turning now to the blood system of Mammals we find that the 

 red blood corpuscles which give the colour to the blood are unlike 

 those of other Vertebrates. They have no nuclei and are bicon- 

 cave, while they are also much smaller and (except in Camels and 

 Llamas) are circular, not oval discs as in all lower Vertebrates. 

 Like Birds, but unlike most Reptiles, the Mammals have a four- 

 chambered heart; the main blood-vessel, the aorta, is supplied by 

 the left systemic arch alone, the right one being cut off from con- 

 nection with it and being represented by the common trunk of the 

 right carotid and subclavian arteries, the so-called innominate 

 artery (Fig. 323), this being exactly the converse of the arrange- 

 ment in Birds. 



The ventral carotid arteries, which we have seen are reduced in 

 Birds as compared with Reptiles and Amphibians, are usually 

 absent in Mammals, though they may exist as quite small vessels 

 but the trachea and other structures in the neck receive blood from 

 the common or dorsal carotid on its way to the head. The sub- 

 clavian artery is of the dorsal type as in Lizards and Amphibians 

 in all Mammals except Cetaceans, in which order the fore-limb or 

 paddle obtains blood by an artery corresponding in origin with 

 that to the fore-limb of Chelonians, Crocodiles and Birds, i.e., a 



