2i6 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



has but five pairs, one pair must be missing. The missing pair is the first, which 

 should supply the first visceral arch; it has disappeared during development, 

 probably because this arch bears no gills. 



Draw the ventral aorta and the afferent branchial arteries showing their 

 relation to the visceral arches. 



6. The efferent branchial arteries and the dorsal aorta. Open the flap 

 already formed of the floor of the mouth and pharyngeal cavities; turn it out- 

 ward and fasten it in that position. The esophagus may be cut to facilitate this 

 procedure. With a forceps strip off the mucous membrane from the roof of the 

 mouth and pharyngeal cavities. In the roof there will now be seen four pairs 

 (dogfishes) or three pairs (skate) of large blood vessels extending from the angles 

 of the gill slits obliquely caudad. They are the efferent branchial arteries and 

 represent the dorsal halves of the aortic arches. Clean away the connective 

 tissue from these arteries so that they are clearly exposed. Trace them to the 

 left and note that they disappear dorsal to the cartilages of the gill arches. 

 Remove these cartilages carefully. This is best done by cutting across them in 

 the visceral arch, grasping the cut end, and loosening the cartilage toward the 

 median dorsal line. After the cartilages are removed trace each of the efferent 

 branchial arteries toward the gills. Note that each is formed at the dorsal angle 

 of the gill slit by the union of two vessels, a smaller pretrematic branch which 

 comes from the demibranch on the anterior face of the visceral pouch and a 

 much larger post-trematic branch which comes from the demibranch on the 

 posterior wall of the visceral pouch. Note the small vessel which runs from each 

 gill filament into the pre- and post-trematic branches. In the skate the first 

 two efferent branchial arteries unite to one so that there are but three pairs of 

 main vessels in the roof of the pharyngeal cavity. 



Next, dissect on the right side, which has been kept intact, in order to see 

 the full course of the efferent branchial arteries. Remove the mucous membrane 

 from the floor of the mouth and pharyngeal cavities, thus exposing the ventral 

 portions of the gill arches. Remove these cartilages carefully without disturbing 

 any of the arteries, and also remove the cartilage from the full length of the 

 visceral arches. It will now be seen that the pre- and post-trematic branches 

 are united at their ventral ends so that they form a complete loop around each 

 gill cleft. Note further that the post-trematic branch on the anterior wall of 

 each visceral arch is connected with the pretrematic branch on the posterior 

 face of the same arch by means of cross-branches, about three in number in the 

 spiny dogfish, one in the smooth dogfish and skate. Thus each efferent branchial 

 artery collects from several demibranchs. 



From the ventral ends of some of the loops formed by the pre- and post- 

 trematic branches vessels arise on each side and proceed posteriorly in the floor 

 of the pharyngeal cavity, ventral to the cartilages of the gill arches, to supply 

 the pericardial cavity, wall of the heart, and visceral muscles. These vessels 



