THE DOGFISH 17 



peritoneal canal (p. 14) and see where it enters the pericardial 

 cavity. 



Sketch the heart in diagrammatic sagittal section, X2 f 

 bringing out these points and showing the varying thickness 

 of the walls. 



The pharynx is that portion of the alimentary canal which 

 serves both for respiration and for the passage of food. In the 

 dogfish, therefore, it extends from the spiracle to the last gill-cleft. 



Cut from the angles of the mouth back through the middle of 

 the gill-slits, removing the whole lower jaw and floor of the gill- 

 region. In the roof of the mouth find the inner end of the spiracle. 

 In the cut section make out various skeletal elements (cartilages) 

 as follows: In front the arch of the jaws (mandibular arch); 

 just behind this the hyoid arch; and a branchial arch in each 

 gill-septum. Connected with hyoid and branchial arches are 

 slender branchial rays. Find also in the pharyngeal wall the 

 blood-vessels of each septum. 



Draw the roof of mouth and pharynx and the cut surface, 

 bringing out these points as well as the relation of gill-folds to 

 the septa and the short finger-like processes, the gill-strainers, 

 on the inner margins of the gill-clefts. 



Slit the skin of the roof of mouth and pharynx in the middle 

 line and remove it from either side, exposing the efferent branchial 

 arteries running obliquely inward and backward from the septa. 

 Dissect them free from the investing tissue and trace them into 

 the septa, as well as backward to form a large vessel, the dorsal 

 aorta. Running forward from the anterior end of the dorsal 

 aorta, between the efferent branchials are a pair of vertebral 

 arteries. How do they end in front? How many efferent 

 branchials in all. How is this reconciled with the number of gills? 

 Are the arteries in the gills connected by cross-trunks? How 

 is blood brought back from the last demibranch? 



From the anterior part of the first efferent branchial a com- 

 mon carotid artery extends forward and medially, soon dividing 

 into an external carotid artery, which proceeds forward to supply 

 the side of the head, and an internal carotid which continues to 

 the median line to join its fellow of the opposite side, the com- 

 mon trunk thus formed passing upwards through the floor 

 of the cranium to supply the brain. 



Add all of these vessels to the drawing of the arterial system. 



