THE ELASMOBRANCH FISHES 161 



imposed iii)Oii it, its walls have become greatly thickened. Inside, the lining 

 has become thrown into numerous irregularities and is richly i)rovided with 

 supporting chordae tendineae stretched from the walls of the ventricle to the 

 auriculoventricular valves. These tendinous cords prevent the valves from 

 being forced into the auricle ui)on the contraction of the ventricle. 



The conus arteriosus connects the ventricle and the ventral (ascending) 

 aorta (vm.). Its walls are muscular so that it serves to keep the blood sent out 

 from the ventricle at a more constant pressure. A longitudinal section through 

 the conus of Heptanchiis shows its thickened walls and, on the sides of the 

 lumen, a series of pocket-like valves {v.c, fig. 150b), which prevent blood 

 from reentering the ventricle. These are arranged in three longitudinal rows, 

 one dorsal and two ventrolateral in position. The anterior and much larger 

 valves in each row are located just back of the exit of the most posterior 

 afferent branches of the ventral aorta. Following these are other small valves, 

 behind which are three valves of medium size, in each row. 



ARTERIES 



Ventral Aorta 



The ventral aorta {v.a., fig. 150a) continues forward from the conus and, in 

 the region back of the mandibular symphysis, divides into right and left 

 halves. Along its course it gives off paired branches, the afferent arteries 

 {hy.af., and hr.af}~'^, fig. 150a) which distribute blood to the gills. 



AFFERENT ARTEEIES OF ADULT 



In Heptanchus these branches are seven in number. An anterior pair, con- 

 sisting of the hyoidean afferent (hij.af.) and the first branchial afferent 

 {hr.af}), arises from a common trunk formed by the bifurcation of the an- 

 terior end of the ventral aorta. The first or hyoidean supplies the hyoidean 

 demibranch, entering in front of the first gill pocket. The second enters the 

 first whole gill l)etween the first and second pockets, supplying both of its 

 demibranchs. Following these on each side are given off the second to the sixth 

 branchial afferents, the last two of which arise close together from the ventral 

 aorta (fig. 150b, ap.). The second to the sixth branchial afferents enter and 

 supply the fourth-fifth, sixth-seventh, eighth-ninth, tenth-eleventh, and 

 twelfth-thirteenth demibranchs. Branches from the afferents break up into 

 smaller and smaller arterioles and finally as capillaries supply the filaments 



of all the gills. 



EFFEREN T-COLLECTOES 



From the capillaries the blood passes down the filaments into efferent-col- 

 lectors. If the most anterior gill pocket of an injected specimen be opened, as 

 is the second pocket in figure 142, an efferent-collector artery would be seen to 

 drain the demibranch in front of the cleft, the hyoidean demibranch (see also 



