APPENDIX. 469 



muscles so as to fully expose the pericardial cavity. Dis- 

 sect away the superficial layer of muscles from the region 

 between the pericardial cavity and the lower jaws, so as 

 to expose a pair of longitudinal muscles running forward 

 side by side from the pericardial region to the basi-hyal 

 cartilage. On slightly separating these the thyroid gland 

 can be seen in the middle line : it is a useful land-mark, 

 as it lies just ventral to the anterior T-shaped end of the 

 truncus arteriosus (fig 92, p. 177). Before going further, 

 it is necessary to stretch the floor of the pharynx by 

 pushing something down the throat : a bone-forceps does 

 very well, or a carrot may be cut to a suitable shape and 

 pushed in at the mouth. You next proceed to carefully 

 dissect away the two longitudinal muscle-bands above 

 mentioned, taking great care not to cut the arteries that 

 lie just beneath them. These arteries (first and second 

 afferent branchial) are thus exposed, and the first can 

 be traced very easily along the hyoid arch, its branches to 

 the gill-filaments of the first demibranch being well 

 shown. The truncus arteriosus should be traced from its 

 anterior end back to the pericardium, not vice versa. When 

 traced half-way it may be left until the other afferent 

 branchials have been found, as they can easily be by 

 dissecting just anterior to the inner ventral corner of each 

 of the three posterior gill-pouches. Each artery at this 

 point lies within a small cavity (probably homologous 

 with the branchial coelomic canals of Amphioxus) and so 

 is less likely to be cut than if searched for at its origin 

 from the truncus. When once the level at which the 

 truncus and afferent arteries lie is clearly realized, all 

 muscles lying ventral to that level may at once be sliced 

 away. The rest of the dissection consists in tracing out 

 and cleaning the various arteries. The ventral portions 

 of the gills may be cut away. 



Another dissection that should not be omitted is that of 

 the internal ear or membranous labyrinth of the dogfish. 

 The surface of the auditory capsule should first be scraped 

 thoroughly clean, and then the cartilage cut away piece- 

 meal so as to leave uninjured the semicircular canals, etc., 

 which are embedded in it. 



The odour of the dogfish is unpleasant and clinging. 



