102 THE FROG CHAP. 



III. The chief arteries may now be followed out (p. 80, 

 Fig. 20) : 



Note the carotid, the systemic, and the pulmo-cutaneous 

 trunk, arising from the conus arteriosus, and then trace 

 each of these out as follows : 



1. The carotid trunk gives off a lingual artery and is con- 

 tinued into the head as the carotid artery, having at its 

 origin the carotid labyrinth. 



2. The systemic trunk unites with its fellow to form the 

 dorsal aorta, first giving off vertebral, subclavian, and 

 cesophageal arteries. From the point of union of the two 

 systemic trunks arises the splanchnic or c&liaco-mesenteric 

 artery. After following this out to its distribution, remove 

 the alimentary canal as directed on p. 76, when the following 

 branches of the dorsal aorta will be more plainly seen : the 

 renal, spermatic or ovarian, and iliac arteries. 



3. The pulmo-cutaneous trunk divides into a pulmonary 

 artery, passing along the outer side of the corresponding 

 lung, and a cutaneous artery. 



Sketch the heart and chief arteries, and then make out 

 and sketch the renal portal system (p. 101), if you have not 

 already done so. 



IV. 1 Cut out the heart of a frog preserved in formaline, 

 taking great care not to injure it. Fasten it down in a 

 dissecting -dish, with the ventral surface upwards, by sticking 

 very small pins through the arteries and veins not through 

 the" heart itself. Pinch up the ventricle with fine forceps, 

 and with small scissors gradually snip away its ventral wall, 

 noting that it is a hollow structure with thick, spongy walls 

 and a small cavity, which will probably be full of clotted 

 blood. Wash this out and then proceed to open the auricles 

 in a similar way, and to wash out the blood they contain. 

 Observe the right and left auricles, separated by a partition. 

 Slit open the conus arteriosus, and continue the cut forwards 

 to the origin of the main arteries. Examine with a lens and 

 make out (p. 87, Fig. 22) : 



1. The auricula-ventricular aperture and its valves. 



2. The longitudinal valve and the small semilunar valves in 

 the conus arteriosus. 



3. The origins of the carotid and systemic trunks from the 



1 On account of its small size, the examination of the structure 

 of the frog's heart is somewhat difficult, and the student is 

 advised to dissect first the heart of a larger animal, such as a 

 dogfish (pp. 449 and 477). 



