490 ANATOMY OE VEETEBRATES. 



Tliis aorta extends beneath the bodies of the vertebra3 along the 

 abdomen and through the hasmal canal to the end of the tail. In 

 many Cyprinoid fishes it dilates beneath each abdominal vertebra 

 into a sinus. It gives off intercostal arteries, which in many 

 adult fishes become fewer in number than the intercostal spaces ; 

 it sujjplies numerous small branches to the kidneys. In the 

 Syngnathi the aorta grooves the kidney in its course, and in the 

 Anchovy sinks into the renal substance. The first principal 

 visceral branch is the ' cceliac ; ' which sometimes, as in the 

 Burbot, is sent off from the posterior part of the ' circulus 

 aorticus,' and in some Sharks by two trunks from the same part. 

 The next branch is a posterior mesenteric, which varies in size 

 according to the extent of the intestinal canal supplied by the 

 cocliac. Between these, in some fishes, the brachial arteries are 

 sent off from the abdominal aorta : these vessels in the large- 

 finned Tor])edos and Chimrera! have a partial investment of 

 muscular fibres, like secondary builds, but without any valvular 

 structure to give effect in onward flow to their action.' 



In the Porbeagle Shark (yLcnnna cornuhica) the two cceliac 

 arteries each split into a bundle of small arterioles, which inter- 

 lace Avith a similar resolution of the hepatic veins to form a mixed 

 fasciculate ' plexus mirabilis ' between the pericardial septum and 

 the liver. The arterial blood is collected again into a trunk on 

 the outer side of each plexus ; and is distributed by the ramifi- 

 cations of those trunks in the ordinary way to the stomach and 

 intestines.^ The arterial branches to the spiral valve in the Fox 

 Shark are rcmarkal)lc for the rich bundles of twigs hj which they 

 distriliute the blood to that production. In the Mediterranean 

 Tunnies (Thi/nnus and A/i.vis) the branches of the ci"eliaco-mescn- 

 teric artery sent to the stomach, the pancreas and the intestines, 

 severally split up into similar fasciculate plexuses, which are 

 interlaced with corresponding plexuses of the veins from those 

 viscera prior to the formation of the portal trunk. 



But the most common modification of the visceral vascular 

 system is the sudden division and termination of a branch, usually 

 of the gastric artery, in a small body chicfiy composed of the 

 cellular beginnings of the returning veins, forming the vaso- 

 ganglion so constant in all higher Vertebrates, and called the 

 ' spleen,' fig. 276, ;/ ; fig. 281, r/. It is not jiresent in the Lancelet ; 

 and the gland-like Imdics near the cardla in tlic C'ycloftomes, and 

 near the pylorus in tlio Lc)iidosiroii, whicli some have called 

 ' spleen,' are more like the recognised remnants of the vitellicle in 

 Osseous Fishes, where a trite spleen is actually present. The 

 ' -xcvui. 2 XXI. !>. 'J'.l, pi. r>. 



