16 GUIDES FOR VERTEBRATE DISSECTION 



(afferent branchial arteries) and trace them into the gill-septa 

 of the uninjured side. How many of these afferent branchials 

 do you find? How many trunks leave the aorta on either side? 

 Is there an artery to the posterior side of the fifth gill-cleft? How 

 does this agree with the distribution of the demibranchs? Trace 

 twigs from the afferent branchials into the gill-filaments. 



Add the ventral aorta and the afferent branchial arteries 

 to the drawing of the arterial circulation. 



Cut through the sinus venosus just behind the auricle, leaving 

 it attached to the septum transversum. Tip the heart forward so 

 as to examine the auricle. Is the heart bound dorsally to the 

 pericardium? Notice on the ventral wall of the truncus a pair 

 of anterior coronary arteries running back to the ventricle, and 

 on the dorsal side of the truncus a similar pair of posterior 

 coronary arteries. Which are the larger? Are the posterior 

 pair distributed to the auricle or to the ventricle? Trace the 

 coronaries forward into the region in front of the pericardium and 

 thence to the vessels of the gill-slits. With which of these are the 

 coronariet connected? 



Cut through the truncus just inside of the pericardium and 

 remove the heart from the body. Sketch it from the side, insert- 

 ing the coronary arteries. Then, placing it in the normal position 

 (i.e., with auricle uppermost) lay open the auricle from the dorsal 

 surface and wash out the contents. Notice the projecting mem- 

 branous folds the sinu-auricular valves bounding the opening 

 between the sinus and the auricle. In the floor of the auricle is 

 the opening (auriculo-ventricular canal) into the ventricle. In- 

 sert the scissors in this opening and split the ventricle longitudi- 

 nally, carrying the incision through the truncus arteriosus. 

 Notice the relative thickness of auricle and ventricle. Also see 

 the auriculo-ventricular valves around the canal, and also the 

 fibres passing from the wall of the ventricle to the free margins 

 of the valves. Careful examination will show that these fibres 

 have a muscular enlargement (musculus papillaris) at the base, 

 the rest (chorda tendiniae) being tendinous in character. See 

 the pocket-like valves (how many rows and how many in a row?) 

 in the region (conus arteriosus) where the truncus arises from 

 the ventricle. These, like the other valves, prevent a back- 

 flow of the blood. The muscular portion of the truncus in front 

 of the conus is the bulbus arteriosus. Probe the pericardio- 



