12 GUIDES FOR VERTEBRATE DISSECTION 



and truncus? Farther forward the walls of the truncus are 

 thickened and muscular, forming a bulbus arteriosus. How 

 many openings do you find from the auricles into the ventricle? 

 Cut into the auricles: are the chambers entirely distinct from 

 each other? Do both connect with the sinus venosus? If not, 

 which one, right or left, has such a connection? 



Make a diagrammatic sketch of the heart, showing the relations 

 of the chambers. 



The Afferent Branchials. Follow the truncus arteriosus 

 forward in front of the pericardium by removing the covering 

 muscles and notice that it branches almost immediately into 

 right and left trunks, the portion before the branching being all 

 that remains of the ventral aorta. Trace out one of the branches 

 and see how it divides into afferent branchial arteries. How 

 many are there of these? and what relation does each bear 

 to branchial arches, gill-slits, and external gills? Cut off one of 

 the gills and see the vessels in it. How would you describe their 

 position? 



Add the heart and the afferent branchial arteries to the sketch. 



The Mouth. Insert strong scissors at the angle of the mouth, 

 the points of the blades extending just dorsal to the external 

 gills; cut, and lay back the floor. Make a sketch of the roof of 

 the mouth showing the distribution of teeth in three pairs of 

 patches upon the premaxillaries, vomers, and the anterior end 

 of the palatopterygoids. Find the internal nares (choanae) by 

 probing from the external nostrils. 



In the floor of the mouth see the short tongue. Recognize in 

 it, by feeling, the hyoid cartilage and extending outwards and 

 backwards from this the branchial cartilages. What relation 

 do these bear to the gill-slits? Insert gill-clefts and teeth in a 

 sketch of the floor of the mouth, and, by dotted lines, the posi- 

 tions of the cartilages. Note the projections on the margins of 

 the gill-clffts; to what in fishes are they to be compared? Do 

 you find internal gills? In the median line behind is a narrow 

 slit, the glottis. Insert a blowpipe in it and inflate. What 

 happens to the lungs? Notice the change in the lining of the 

 throat just behind the glottis, indicating the line between pharynx 

 and oesophagus. 



Indicate these features in the sketch of the floor of the mouth. 



Slit up the oesophagus, lay open the roof of the mouth cavity 



