186 THE ELASMOBRANCH FISHES 



more, upon the posterior fusion of right and left cerebrals a midventral artery, 

 the basilaris (hs.), is produced, which supplies the medulla of the brain and as 

 the spinalis (a.sp., fig. 172a) continues ventrally down the spinal cord. The 

 spinalis will again be considered in our description of the arteries of the spinal 

 cord. In Raja (fig. 172c) the posterior cerebrals form broad arches on the 

 brain stem before uniting. Here the spinalis is made up of several strands. 

 According to Hyrtl it receives the myelonal vessel (ml.) from the united first 

 and second efferents, as was seen in figure 170c. 



Arterial Supply to Trunk 



DORSAL AORTA 



The dorsal aorta in all the Elasmobranchs is supplied with blood from the 

 branchial efferent arteries; it extends through the body and is continued into 

 the tail as the caudal aorta. It arises in the embryo as a pair of arteries. Evi- 

 dence of this condition may be absent in the adult, but in forms like Acanthias 

 and Zygaena the anterior part of the aorta, "the vertebral," to which the 

 second embryonic aortic arch is attached, indicates the paired condition. In 

 Acanthias the paired aortae {da}, fig. 170) , as widely separated arteries, pass 

 forward and are joined by the hyoidean efferents; while in Zygaena (fig. 

 170b) the two are fused far forward. In the adult ray, on the contrary (fig. 

 170c), all indication of the paired aortae is lacking. In the region of the trunk 

 the paired embryonic aortae early fuse into a single median dorsal aorta, 

 which is the source from which many vessels arise. These arteries, as in Hep- 

 tanchus, spring from the aorta either as paired or as unpaired vessels. 



UNPAIRED ARTERIES 



The unpaired arteries in anteroposterior direction are the coeliac axis {ce., 

 figs. 173-175), the superior mesenteric {s.m.), and the so-called inferior mes- 

 enteric {i.m.) arteries. When a fourth is present, as is usual in the sharks, it 

 is due to a failure to form a superior mesenteric, the two branches of which 

 arise separately from the dorsal aorta. 



COELIAC AXIS AND ITS BRANCHES 



The coeliac axis {ce., figs. 173-175) arises posterior to the union of the fourth 

 branchial efferents and behind the paired subclavians. In some forms this is 

 a relatively short trunk {Mustelus), while in others it is long {Acanthias) . 

 Normally as in Heterodontvs {ce., fig. 174) it supplies the gonad and then 

 divides into two parts, one of which, the gastrohepatic {gh.), supplies the 

 stomach and the liver; the other, the anterior intestinal, {a.i.a.), passes back- 

 ward to the region of the intestine. 



The gastrohepatic may be a fairly well developed segment as in Triahis (fig. 

 173a). It is, however, usually a short trunk as in Acanthias; or it may be en- 



