548 



CHORD AT A. 



The swim bladder of fishes and the lungs of most amphibia are smooth- 

 walled sacs, but in some have greater respiratory surface since folds ex- 

 tend into the central space. This peripheral folding increases in the rep- 

 tiles at the expense of the central chamber, this in some being completely 

 divided by the partitions, which extend inwards from the walls to the 

 bronchus. In the mammals a central chamber is lacking; the bronchi 

 extend into the lungs, brandling again and again to the fine bronchioles 

 which give off alveolar ducts lined with minute air cells or alveoli. 



The circulatory apparatus is easily derived from that of annelids, 

 and, like it, is completely closed. In the annelids (p. 307, figs. 



272, 275, 276) above and below the 

 digestive tract is a longitudinal blood- 

 vessel, these being connected in each 

 somite by loops which pass around 

 the intestine. The vertebrate scheme 

 varies in the development of a heart 

 in the ventral trunk (the dorsal of 

 the annelid). In the lower verte- 

 brates, the fishes (figs. 65, 597), the 

 heart lies close behind the gills and 

 sends to them the blood which it 

 receives from the body. Hence, like 

 the whole ventral trunk, it carries 



FIG. 579.-Lungs of man, ventral view. VellOUS bl d - SinCe the anterior 



!ta!&8^Afi:% lo P s > the m arteries > P ass 



viding below into the two bronchi; fV, p mlla fhp dorsal trnnlr 

 Z, position of diaphragm; I,*, 3, Sa, tn<3 111S > l UnK > 



globes of right and left lungs. collects from these, must contain 

 oxygenated blood, which is sent by the carotids to the head, and 

 by the dorsal aorta and the vascular loops to the body. It thus 

 becomes venous and flows back into the ventral trunk. 



This scheme of circulation in fishes needs further description. 

 The heart, a strong muscular organ enclosed in a pericardium, con- 

 sists of two parts, auricle and ventricle, separated by valves. The 

 trunk (ventral aorta) arising from the auricle is arterial and cor- 

 responds to the ascending aorta and pulmonary artery of man. 

 The arterial arches of the gill region which arise from it pass di- 

 rectly into the dorsal vessel only in young fishes (fig. 597); later 

 they furnish the branchial circulation of gill arteries, gill capillaries, 

 and gill veins (fig. 65). The dorsal trunk is the dorsal aorta 

 (aorta descendens) ; the ventral trunk, which only occurs in the 

 embryo, is the subintestinal vein, from which the portal vein arises. 

 To this are added a system of paired veins, consisting of Cuvierian 



