102 THE FROG chap. 



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

 first giving off vertebral, subclavian, and cesophageal arteries. From the 

 point of union of the two systemic trunks arises the splanchnic or 

 caliacoviesenteric 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 cruai ian, and iliac arteries. 



3. The puliiw-cutaneous trunk divides into & 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. loi), 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 out 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 auriculo-ventricular aperture and its valves. 



2. The longitudinal valve and the three small semilunar valves in the 

 conus arteriosus. 



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

 aortce, and the small aperture leading into the pulmo-cutatieotis trunks. 



4. The sinu-auricular aperture and its valves. 



5. The aperture in the left auricle leading into the pulmonary veins. 

 Sketch. 



Turn over the heart, so that its dorsal surface is upwards, and cut away 

 enough of the dorsal wall of the sinus venosus to show the sinu- 

 auricular aperture from the other side. 



' 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 n. larger animal, such as a dogfish or a rabbit. (See 

 Part II.) 



