80 THE FROG CHAP. 



(Figs. 3 and 4), there is seen in the dorsal part of the 

 pericardial cavity a thin-walled chamber (Fig. 21, s. v) 

 of a dark colour, connected with the right side of the 

 auricular division. This is the sinus venosus. 



The Arteries. The two branches of the conus arteri- 

 osus just referred to soon branch again. Each divides 

 into three vessels, often spoken of as arterial arches, 

 called respectively the carotid trunk (Fig. 20, car. tr), 

 the systemic trunk(syst. ^),and the pulmo-cutaneous trunk 

 (pul. cu. tr}. All these conform to the definition of an 

 artery given on p. 27, i.e., they are stout, elastic vessels, 

 containing little blood after death, and not collapsing 

 when empty. 



The carotid trunk divides immediately into two, a 

 lingual artery (Ig), which can be traced to the tongue, 

 and a carotid artery (car], which branches repeatedly, its 

 ultimate ramifications going to various parts of the 

 head. At the origin of the carotid is a little rounded 

 mass with a sponge-like interior, the carotid labyrinth 

 (car. gl). 



The systemic or aortic trunk extends outwards, in 

 contact with the gullet, then sweeps upwards, back- 

 wards, and inwards i.e., towards the middle line and 

 finally joins with its fellow of the opposite side to form 

 a single median vessel, the dorsal aorta (Figs. 4, 5, and 

 20, d. ao), which passes backwards just beneath the 

 vertebral column and between the kidneys. 



As it sweeps round the gullet, the systemic trunk 

 gives off a vertebral artery (Fig. 20, vert) to the vertebral 

 column and part of the head, a subclavian artery (scl), 

 passing into the fore-limb as the brachial, and an 

 cesophageal artery (ces] to the gullet. 



From the point of union of the two aortic trunks 

 springs a single splanchnic or cceliaco-mesenteric artery 

 (ccel. mes) ; it divides into several branches, which are 



