ii8 ZOOTOMY. 



The hyoidean (§ 175) and two first efferent branchial arteries (§ 15b) 

 anastomose at their ventral ends, and from the junction of the two 

 latter an artery is given off on each side, which meets with its fellow 

 below the ventral aorta to foi-m an azygous vessel, which is best seen at 

 this stage. This soon diVides into two branche?, one of which passes 

 above, the other below, the pericardium, supplying the muscles in 

 these regions, and giving off several very small coronary vessels to the 

 heart (Fig. 32, co). The ventral branch eventually divides into two, 

 which supply the pelvic fins (pi). 



147. The afferent branchial arteries (Fig. 32, a.br'-, 

 a.bt^), four on each side, given off laterally from the ventral 

 aorta to the four pairs of gills, and passing dorsalwards 

 along the posterior face of the corresponding branchial 

 arches. 



14S. The inferior jugular vein (Fig. 32, i.j't/), an unpaired vein 

 bringing the blood from the lower parts of the head : it passes through 

 the pericardial cavity to the dorsal side of the heart, and opens into 

 the right precaval. 



XXVIII. Remove the heart and observe again the 

 relations of its parts (§ 115); then open its 

 various chambers by the removal of the left 

 wall of each. This is best done under water. 

 Observe 



149. The thin smooth walls of the sinus venosus. 



150. The sinu-auricular valve, between the sinus 

 venosus and the auricle; it consists of two membranous 

 flaps, respectively dorsal and ventral in position. 



151. The thin walls of the auricle, strengthened by a 

 network of interlacing muscular bands, the musculi 

 pectinati. 



152. The auriculo-ventricular valve, guarding the 

 round aperture between the auricle and the ventricle ; it 

 consists of two flaps, respectively anterior and posterior in 

 position. 



