THE FROG 61 



The spiral fold or valve within the conus is a flap of membrane 

 running obliquely and spirally forwards, practically dividing its 

 lumen into two passages. It begins on the ventral side near the 

 right semi- lunar valve of the posterior series, and, for the most part, 

 attached on the dorsal side with its ventral edge free, it ends on the 

 dorsal side of the conus near the large right semi-lunar valve. 

 Owing to the way in which it is attached, the cavity of the conus 

 is divided into a right channel, the cavum aorticum, which leads 

 off directly from the ventricle and a left channel, the cavum pulmo- 

 cutaneum, not directly continuous with the ventricle. 



The truncus is divided by a horizontal septum into dorsal and 

 ventral compartments. The dorsal chamber at the one end com- 

 municates with the cavum pulmo-cutaneum by an aperture guarded 

 by one semi-lunar valve, and at the other passes over on each side 

 into the pulmo-cutaneous artery. The ventral chamber is con- 

 tinuous with the cavum aorticum, and is further subdivided by a 

 median vertical partition, the septum medium, into a right vacity 

 which leads into the right systemic arch, "and also the two carotid 

 arches and a left chamber which leads into the left systemic arch. 



Arterial System. 



The arteries of the body, forming the arterial system, all 

 arise from the three arches coming off from the truncus arteriosus, 

 namely, the carotid, the systemic and the pulmo-cutaneous. While 

 still united in a common trunk they pass outwards round the 

 oesophagus for a short distance before dividing into separate vessels 

 The Carotid Arch, soon after becoming separate, gives off a 

 branch, the external carotid (often termed the lingual), which supplies 

 the tongue, lower jaw and hyoid apparatus, and then swells out to 

 form, an enlargement, the so-called carotid " gland," in which the 

 carotid breaks up into a considerable number of small vessels which 

 reunite to form the internal carotid artery. This " gland " is the 

 remains of a vascular connection present in the tadpole, and 

 it is suggested by some authorities that it plays a part in the 

 regulation of the blood pressure in the carotid arch. The internal 

 carotid runs outwards and upwards to the base of the skull, where 



applied in the frog to that part of the heart marked off at each end by the 

 semi-lunar valves. Its walls are composed of cardiac muscle, it is rhythmically 

 contractile, and it is homologous with the similarly named portion of the 

 dogfish heart. For some reason or other it has been erroneously termed in 

 some books, truncus arteriosus. The truncus arteriosus (pylangium), however, 

 is merely the terminal chamber, and represents in a very abbreviated form the 

 Ventral aorta (truncus arteriosus or truncus aortae) of the dogfish. The posses- 

 sion of a short truncus arteriosus makes the heart of Rana intermediate between 

 that of Scy Ilium, where it is long, and that of the rabbit, where it is absent 

 altogether. 



