102 THE FROG CHAP. 



2. The sysleinic trunk unites with its fellow to fuim the dorsal aorta, 

 first giving off vertebral, siiljclavian, and asophagea/ arteries. From the 

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

 eceliaco mesenteric artery. .After following this out to its distribution, 

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

 branches nf the dorsal aorta «ill be more plainly seen ; — the renal, 

 spenuatit or oi'ai ian, A\\^ iliac arte) ies. 



3. The piiliiio-ciitancous trunk divides into 2. pubiwnary artery, passing 

 along the outer side of the corrcy>ponding Umg, 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 \entral 

 surface upwards, b)- sticking ver)' small pins through the arteries and 

 veins — not through the heart itself. Tinch up the \entricle with line 

 forceps, and with small scissors gradually snip away its ^X'ntral wall, 

 noting that it is a hollow structure with thick, si:)ongy walls and a small 

 cavity, which will |)robably be full of clotted blood. Wash this out, 

 and then proceed to open the auricles in a similar wa)-, and to wash out 

 the blood they contain. tjbser\e the right and /(// auricles, separated 

 by a partition. Slit open the eonus arteriosus, and continue the cut 

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

 make out (p. S7, Fig. 22) ; — 



1. The auricido-i'cntricular apeiiurc and its valves. 



2. '\\\^e longitudinal valve and the three small semilunar valves in the 

 conus arleri<.»sus. 



3. The origins of the LCirotid and systeiniL trunks from the biilbus 

 aortic, and the small aperture leading into the pulnio-i utancons trunks. 



4. The sinu-anricitlar aperture and its vahes. 



5. The aperture in tlie left auricle leading into tlie 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 \enosus to sliow the sinu- 

 auricular aperture from the other side. 



' On acc.unt of its small size, the examinalion of the slrnrlure of the 

 bog's heart is somewhat difficult, and tlie student is ad\iseil to di.s.sect 

 lirst the heart of a larger animal, such as a dogfish or a rabbit. (See 

 Tart II.) 



