xn VASCULAR SYSTEM 333 



manner. On each side, the longitudinal commissural vessel, 

 which connects the ventral ends of the arterial loops surrounding 

 the different gill-clefts, gives origin to a series of pairs of short 

 transverse vessels, and by their union these combine to form a 

 median longitudinal hypobranchial artery which lies beneath the 

 ventral aorta. From the hypobranchial artery are derived the 

 coronary arteries for the heart ; and from the same artery, or 

 from its lateral connexions with the longitudinal commissural 

 artery, and, in the case of the hyoidean hemibranch, from the 

 mandibular artery, are derived the various nutrient vessels for 

 the gills. 



The arteries for the trunk, and for the pectoral and pelvic limbs, 

 arise in succession from the dorsal aorta. The first of the series 

 is the subclavian artery, which has its origin from the aorta close 

 to the dorsal extremities of the fourth pair of epibranchial arteries. 

 Each subclavian artery gives off a brachial artery to the pectoral 

 fin, and is then continued forwards as a lateral hypobranchial 

 artery, which, with its fellow of the opposite side, eventually 

 becomes continuous with the hinder end of the median hypo- 

 branchial artery. Behind the subclavian artery there is a 

 median coeliaco-mesenteric artery, the various branches of which 

 are distributed to the liver, stomach, and intestine. A lieno- 

 gastric artery supplies the pancreas and spleen, and also sends 

 branches to the stomach. In addition, there are also arteries for 

 the gonads, numerous segmental arteries for the myotomes, and 

 renal arteries for the kidneys. Finally, the aorta gives off a 

 pair of iliac arteries for the pelvic fins, and then enters the haemal 

 canal as the caudal artery. 



The more important differences in the arterial system of the 

 Holocephali and the Teleostomi relate to (1) the absence of the 

 posterior efferent branchial artery in each branchial arch ; (2) 

 modifications dependent on the condition of the spiracular and 

 hyoidean hemibranchs, and the mode of origin and the course of 

 their afferent and efferent vessels ; and (3) the source from 

 whence the air-bladder derives its blood when that organ is 

 present. 



(1) The branchial arterial system is somewhat more primitive 

 than in the generality of Elasmobranchs.^ There are no complete 

 vascular loops round the gill-clefts, and the blood from the two 



1 Of. footnote to p, 332. 



