AMPHIBIA. 197 



edge. The passage on the left of this flap leads to a small pulmo- 

 cutaneous aperture just behind the anterior valves, the passage 

 on the right into the synangium. This is continuous on either 

 side with a systemic arch and in front two small carotid aper- 

 tures, placed on a small elevation, open out of it. 



Arteries (Fig. 54). From the truncus arteriosus two ap- 

 parently single trunks arise, one on either side, and form with 

 it a Y-shaped figure. Each of these trunks is in reality triple, 

 its cavity being divided by two longitudinal partitions into 

 three cavities anterior, middle, and posterior. These belong to 

 three corresponding aortic arches carotid, systemic, and pulmo- 

 cutaneous, into which the trunk soon splits. Into the first and 

 last of these the apertures of the same name lead, and the 

 synangial cavity is directly continuous with the cavities of the 

 systemic arches. 



(1) The carotid arches supply the brain, orbit, and walls of 

 the mouth-cavity with pure blood. 



Each gives off a small lingual artery to the tongue, dilates into a small, 

 rounded body, the carotid "gland," and then becomes the carotid artery, 

 which divides into , external carotid supplying the orbit, and roof and 

 side-walls of mouth-cavity ; /3, internal carotid artery supplying the brain. 



(2) The systemic arches supply the rest of the body with 

 partially purified blood. The arch of each side runs upwards 

 and backwards, giving off branches to the larynx, oesophagus, 

 fore-limb, and other parts, and uniting with its fellow, just 

 ventral to the backbone, near the front ends of the kidneys, to 

 form the dorsal aorta, a median trunk running back in the 

 subvertebral lymph sinus. The dorsal aorta gives off branches 

 to the viscera and body-walls, ultimately dividing, at the pos- 

 terior end of the body, into two iliac arteries, continuations of 

 which run into the hind-limbs. 



Chief Branches. (1) From each arch in front of dorsal aorta (a) laryn- 

 geal to larynx ; (6) one or two ozsophageal to oesophagus ; (c) occipito- 

 vertebra 1 , dividing into () occipital for side of head and jaws, (/3) vertebral, 

 running along one side of the vertebral column, supplying adjacent 

 muscles and sending branches to the spinal cord through the interver- 

 tebral foramina ; (d) subclavlan to shoulder and fore-limb. . (2) From dorsal 

 aorta (a) cceliafo-mcsenteric, coming from near junction of two arches, 

 or from left arch before the union. It divides into a, cceliac artery, 

 (which has two branches, 1, gastric artery to stomach, 2, /lepntic artery to 

 liver and gall-bladder), and ft, mesenteric artery, which breaks up intc 



