65 



The truncus arteriosus is a cylindrical tube somewhat 

 swollen as it lies upon the auricles. The truncus soon divides 

 into two arches, one passing: to the right, the other to the left. 

 Each arch soon splits into three vessels, the carotid, for the 

 head, the pulmo-cutaneous, carrying- the venous blood to the 

 lungs and skin to be oxyg-enated, and the aorta, which curves 

 around to the back to meet its fellow with which it unites to 

 form the descending- aorta. Lift up the ventricle and make 

 out the following structures : The right and the left superior 

 vena cava, bringing back blood from the head and upper ex- 

 tremities ; the inferior vena cava, appearing just above the 

 liver ; the sinus venosus, (practically a fusion of the venae 

 cavae), the chamber into which the three cavae open. The 

 sinus in turn communicates with the right auricle. 



Carefully slit the heart lengthwise into ventral and dor- 

 sal halves. In the ventricle note the comparatively small size 

 of the cavity and the thick walls ; note the two openings in 

 the ventricular cavity — one from the auricles, guarded by 

 auriculo-ventricular valves, the other continuous with the 

 truncus arteriosus ; note in the truncus at its base near the 

 ventricle three small semilunar valves, also a longitudinal fold 

 or so-called spiral valve. The swollen portion of the truncus 

 is known as the pylanguim ; the distal portion formed by the 

 fusion of the aortic arches is known, as the synangium. 



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Sinus- 



Fig. 2 

 Fig. 2. Lateral aspect of the heart, showing its principal parts and the dis- 

 tribution of the branches of the vagus nerve. 



262. Dissection of the Vagus Nerve. Introduce a 

 glass rod into the frog's throat to distend the parts. Beginning 



