294 COMPARATIVE ANATOMY 



diaphragm of mammals is an important organ in respiration. 

 As long as the thoracic walls are intact, the lungs, which 

 have very elastic and distensible walls, are partly filled with 

 air, and are closely pressed against the thoracic walls. When 

 the diaphragm contracts and the thoracic cavity is enlarged, 

 air rushes down the windpipe and enters the lungs which 

 expand to fill the increased space. When the diaphragm is 

 relaxed, this additional air is expelled. The alternate enlarge- 

 ment and reduction of the cavity of the' thorax is further 

 assisted by the movement of the ribs. The breathing move- 

 ments of birds and reptiles are also effected by the ribs, but . 

 the diaphragm is a characteristic mammalian structure. 



The heart of a mammal is quadrilocular, containing two 

 auricles and two ventricles. The right auricle receives the blood 

 brought back by the superior and inferior vens cavas, and on 

 contraction sends it into the right ventricle. On contraction 

 of the right ventricle the blood is forced through the pul- 

 monary artery to the lungs, whence it is returned by the 

 pulmonary veins to the left auricle. The left auricle empties 

 its contents into the left ventricle, and this, by its contraction, 

 forces the blood through the aorta to the whole body. Thus 

 the right and left sides of the heart are (in the adult) com- 

 pletely separated from one another; the right side contains 

 venous blood, the left side contains the oxygenated blood 

 returned from the lungs. The presence of a four-chambered 

 heart in which the venous blood is kept separate from arterial 

 or oxygenated blood is not characteristic of mammalia, for the 

 same arrangement is found in birds, and among the reptilia 

 in crocodiles. But the arrangement of the aortic arches in 

 mammals is characteristic. In the embryo the walls of the 

 pharynx are pierced by gill-slits. Five are developed on each 

 side — viz. the hyomandibular and four branchial clefts — but 

 only three are in place at a time, the anterior slits closing up 

 before the posterior ones are formed. After the heart is 

 formed, the truncus arteriosus is continued forwards on the 

 ventral wall of the pharynx, and divides into six arches on 

 each side ; the first runs in the mandibular arch, the second 

 in the hyoid, the third, fourth, fifth, and sixth in the corre- 

 sponding branchial arches. As the gill -slits are never 

 functional, the arterial arches do not enter gill -capillaries 

 but simply curve round the oesophagus and unite to form 



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