XXI] 



CIRCULATORY SYSTEM 



537 



-14 



The first of these is called the carotid arch, and is derived from 

 the third arterial arch of the embryo, but unlike its equivalent 

 in Dipnoi it does not communicate with the dorsal aorta. It gives 

 off a lingual artery to the 

 tongue and throat and then 

 passes up round the gullet, 

 to which it gives off some 

 twigs and continuing as the 

 common carotid supplies 

 the upper part of the head 

 and brain. Just after giv- 

 ing off the lingual artery 

 the arch swells up into a 

 little knot, called the 

 carotid gland. In this 

 structure the channel of 

 the artery is broken up into 

 a network of fine passages 

 and its function is believed 

 to be that of holding back 

 the blood from entering the 

 head until, at the close of 

 the contraction of the ven- 

 tricle, the blood has return- 

 ed from the lungs. The 

 second arch, derived from 

 the fourth embryonic arch, 

 supplies most of the blood 

 to the root of the dorsal 

 aorta, and on this account 

 is called the systemic 

 arch. The fifth and sixth 

 embryonic arches in later 

 stages unite on each side 

 into one trunk, which pass- 

 ing round the gullet joins 

 the systemic arch (Fig. 

 262). The fifth arch dis- 

 appears in Molge, as in all higher Vertebrates, but it is retained 

 throughout life in the allied genus Salamandra. From the sixth 

 arch is given off the pulmonary artery which supplies the lung. On 

 this account it is called the pulmonary arch. The subclavian 



FIG. 262. Diagram of arterial arches of Molge, 

 viewed from the ventral aspect. 



i, n, in, rv, v, vi. First to sixth arterial 

 arches. 9. Carotid gland. 12. Lingual 

 (ventral carotid). 13. Common carotid 

 (dorsal carotid). 14. Systemic arch. 

 17. Dorsal aorta. 19. Pulmonary. 



22. Subclavian (dorsal type). 23. Cuta- 

 neous. 24. Coeliacomesenteric. 



