IV. VERTEBRATA. 



483 



gland, from a growing air tube (primary bronchus) If the outpushings remain 

 rudimentary, and the primary bronchus widens, the result is the amphibian 

 lung. If the bronchus remain a canal and continue to develop new outgrowths 

 of the second and third order, the complicated lung forms (2, 3) result. 



The circulatory apparatus is easily derived from that of annelids, and, 

 like it,is completely closed. In the annelids (p. 272) there isa longitudijial 

 blood-vessel and another below the digestive tract, these Ijeing 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 (dorsal 

 of the annelid). In the fishes (ligs. 66, 554), 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 venous blood. Since the 



Fig. 536. — Diagrammaliclong section of lungs (i) of a young salamander; (2) turtle; 

 (3) man; (4), a bronchiole (ft), givingoff several alveolar ducts (g), lined with alveoli (a), 

 and connecUng with infundibula (e). 



anterior loops, the gill arteries, pass through the gills, the dorsal trunk, 

 which collects from these, must contain oxygenated blood, which is sent 

 by the carotid arteries to the head, and by the dorsal aorta and the vascular 

 loops to the body. It there becomes venous and flows back into the 

 ventral trunk and so to the heart. 



This scheme of circulation in fishes needs further description. The 

 heart, a strong muscular organ enclosed in the pericardium, consists of 

 two parts, auricle {atrium) and ventricle, separated by valves. The 

 trunk {ventral aorta) arising from the ventricle is arterial and corresponds 

 to the ascending aorta and pulmonary artery of man. The arterial arches 

 of the gill region which arise from it pass directly into the dorsal vessel 

 only in young fishes (fig. 554) ; later they furnish the branchial circulation 

 of gill arteries, gill capillaries, and gill veins (fig. 66). The dorsal trunk 

 is the dorsal aorta {aorta descendens) ; the posterior ventral trunk, which only 

 occurs in the embryo, is the siihintestinal vein, from which the portal 

 vein, going to the liver, arises. To this are added a system of paired 



