142 ZOOLOGY. 



5. Heart and Arterial Arches. The heart of the frog 

 differs from that of the rabbit in being very imperfectly 

 divided into right and left portions. Only the auricles in 

 fact are paired, there being but a single ventricle into which 

 both auricles open. Two now structures, not recognizable 

 in the rabbit, are also found : a sinus venosus, on the dorsal 

 side, formed by the union of the three caval veins and open- 

 ing into the right auricle ; and a truncus arteriosus, repre- 

 senting the united bases of both the aortic and pulmonary 

 arches of the rabbit, and coming off from the right anterior 

 region of the ventricle. 



The arteries and veins of the frog show an almost perfect 

 bilateral symmetry. The truncus arteriosus runs forwards 

 and soon divides into right and left portions, and each of 

 these soon divides into three, so that instead of two arches 

 on the left side only, as in the rabbit, we have three arches 

 oneach^j^Q. The most anterior of these is called the 

 carotid arch, the second is the systemic arch, and the third 

 the pulmo-cutaneous arch. The two latter obviously answer 

 to the two on the rabbit's left side. There is no connexion 

 between them answering to the ductus arteriosus. Connex- 

 ions of a similar kind do, however, usually occur between 

 the carotid arch and the systemic on both sides ; they are 

 called the " ducts of Botallus." On the carotid arch is a 

 swelling, the " carotid gland," which resembles a compacted 

 mass of small blood-vessels. 



From the carotid arches come off vessels to the whole 

 head region. All the other systemic arteries come from the 

 systemic arches. Each pulmo-cutaneous arch gives off an 

 artery to the lung, and another special artery (unrepre- 

 sented in the rabbit) to the skin. For the details of these 

 vessels the student is referred to fig. 70. 



6. Mechanism of Circulation. In spite of the un- 

 divided condition of the ventricle there is a very fair 

 realization of a double circulation like that of the rabbit. 

 The ventricle has its cavity greatly divided up by strands 

 of muscle crossing it, so that it is more like a spongy 

 solid than a hollow chamber. When the heart beats, 

 the auricles contract together, and the right side of 



