IIEAET AND ARTERIES OF VERTEBRATA. 



579 



the aorta. The "blood which is brought to the gills from the 

 stomach by the branchial arteries is venous blood, for it is returned 

 to the heart from the systemic circulation ; as it passes through 

 the capillary network of the gills it again 

 becomes arterial blood, and passes as such 

 through the branchial veins into the aorta, 

 and so to the systemic circulation. 



The number of branchial arteries given 

 off from the arterial bulb is the same as 

 that of the functionally active gills. It 

 is largest in the Cyclostomata and the 

 Selachii. There are five pairs in the 

 Ganoi'dei also, while in the Osseous Fishes 

 it is only during the embryonic stage that 

 there is a larger number (6 or 7) of arterial 

 arches. The two anterior ones belong to 

 the mandibular and hyoid arches, and have 

 either no relations to the gills, or the gill 

 of the hyoid arches is functional for a time 

 only (opercular gill). When the hinder- 

 most gill, which belongs to the more or less 

 rudimentary last branchial arch is atrophied, 

 the arterial arches are reduced to four, and in some cases to three 

 pairs. 



These branchial arteries are given off in very various ways. They 

 either arise in pairs from the 

 simple primary trunk, which 

 ends when it has given off 



Fig. 325. Diagram of the 



arterial arches. 1 — 5. 



a Branchial artery, a" Aorta. 



c Carotid. 



the last 



pair, 



or 



they 



are 



given off on either side of a 

 common short trunk; this is 

 especially well seen in the 

 posterior branchial arteries of 

 the Selachii, and in various 

 Ganoids and Teleostei ; or, the 

 primary trunk of the bran- 

 chial artery divides at its 

 origin into two equal and 

 lateral branches, from which 

 the various branchial arteries are given off (e.g. Bdellostoma among 

 the Mysinoidea). 



Fig. 326. Head of an embryonic Teleo- 

 stean, with the rudiments of the vascular 

 system (Diagrammatic), a Auricle, v Ven- 

 tricle, abr Branchial artery. c Carotid. 

 ad Aorta, s Branchial clefts, n Nasal pit. 

 sv Sinus venosus. dc Ductus Cuvieri. 



§ 433. 



The appearance of lungs is of the greatest importance in effect- 

 ing a change in these relations, for they produce great changes in 

 the arrangement of the large vascular trunks by taking on the 

 function which was previously performed by the gills. This change 

 affects also the structure of the heart ; the Dipnoi afford an interesting 



