300 ZOOLOGY FOR MEDICAL STUDENTS chap. 



capillary blood-vessels (hence the bright red colour of the gills of a fish). 

 Further the surface has its area increased by its growing out into flattened 

 strap-like projections continuous with the septum along one edge and 

 having the other edge free. These are the respiratory lamellae (Fig. 



124, B, r.l). 



There are numerous lamellae radiating outwards upon each face of 

 each gill-septum. Lamellae are also present upon the anterior wall of 

 the first branchial cleft but there are none upon the posterior wall of the 

 last cleft. 



The septum is continued outwards past the outer ends of the lamellae 

 and is eventually bent in a tailward direction, forming a valvular flap 

 over the external opening of the gill-cleft immediately behind the septum. 

 The effect of the presence of these valvular flaps is, while permitting the 

 exit of water through the external opening of the cleft, to prevent its 

 entrance. 



In front of the series of ordinary gill-clefts is a modified cleft known 

 as the spiracle (Figs. 121 and 124, v.c.T), a short wide tube opening ex- 

 ternally just behind the eye. The study of development shows this to 

 be homologous with the other clefts : it is simply number I of the series : 

 and a few vascular ridges (pseudobranch) upon its anterior wall indicate 

 the persistent remains of the lamellae which were once present. The 

 fact that spiracle and branchial clefts form a continuous series of homo- 

 logous openings is expressed in the common name visceral clefts. The 

 individual visceral clefts are designated by numbers, commencing from 

 the anterior end of the series. 



The live Dogfish keeps pumping sea-water over the respiratory 

 lamellae by rhythmic breathing movements. The pharyngeal region is 

 dilated by the action of appropriate muscles upon the skeleton of that 

 region of the body. During this process water is drawn into the pharyn- 

 geal cavity through the slightly opened mouth and the spiracle, the 

 external openings of the clefts being closed by the pressure of the water 

 against the valvular flaps already mentioned. The pharyngeal region is 

 now rapidly contracted, the mouth being closed and a valve being drawn 

 across the opening of the spiracle, and the water rushes out through the 

 gill-clefts, raising up the valvular flaps as it does so. 



The pharynx is continued by a wide and short oesophagus (Fig. 



125, oes) into the J-shaped stomach (st) the distal 1 limb of which is 

 shorter and narrower than the proximal. At the pylorus, where the 

 stomach passes into the intestine (int) the muscular wall of the alimentary 



'■ In anatomical description the adjectives proximal and distal are used in 

 the sense of " nearer " and " further." 



