XXV] RESPIRATORY MOVEMENTS 655 



right renal vein. The anterior portions of the posterior cardinals, 

 however, persist as the venae azygos and hemiazygos, which 

 on each side receive the veins from the spaces between the ribs 

 the intercostal veins. Only that on the right the vena azygos 

 reaches the ductus Cuvieri (superior vena cava); the left cardinal 

 or vena hemiazygos develops a transverse branch through which 

 its blood joins that of the right cardinal (Fig. 324). 



One of the most interesting peculiarities of Mammals is theiri 

 breathing mechanism. It will be remembered that whereas the 

 Amphibia simply swallow air, in the Reptiles the size of the chest 

 cavity is enlarged by pulling the ribs forward and then separating 

 them, and as the lungs are closely attached to the wall of the 

 chest, they are likewise enlarged and air rushes into them. In 

 Mammals this same mechanism exists, but in addition there is a 

 totally independent means of pumping air into the lung. This is 

 rendered possible by the existence of a diaphragm, a partition 

 convex in front which separates the coelom of the chest from the rest 

 of the body-cavity. This partition is partly muscular, and when the 

 muscle contracts the whole membrane is tightened and necessarily 

 flattens, with the result that the chest-cavity is enlarged and air 

 enters the lungs. The action of the diaphragm in fact is precisely 

 similar to that of the muscular floor of the mantle-cavity of the 

 Snail (see p. 287). The diaphragm is attached ventrally to the 

 xiphoid cartilage, dorsally to the vertebral column, and laterally to 

 the ventral edges of the hinder ribs which do not reach the sternum. 

 By it the coelom of the Mammal is separated into a thoracic division 

 in front and an abdominal one behind. The thoracic division is 

 divided into two pleural cavities, one surrounding each lung,. by 

 the pericardium. Since all the vertebrae which bear recognisable 

 ribs which reach the sternum belong to the thoracic region, they 

 are termed thoracic vertebrae, while the ribless vertebrae of the 

 abdominal region are denominated lumbar. 



In the digestive system the principal peculiarity of Mammals is 

 the high state of development of the salivary glands. These 

 glands are much branched tubular outgrowths of the ectoderm of 

 the mouth-cavity or stomodaeum ; they secrete a fluid which 

 moistens the food and is swallowed with it, thus helping digestion. 

 They are foreshadowed by small glands in Frogs and Snakes, but 

 in Mammals they form four large masses, viz. the sublingual, 

 underneath the tongue,, the submaxillary, under the angle of 

 the jaw, the parotidjust under the ear, and the suborbital inside 



