VASCULAR SYSTEM OF FISHES. 271 



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 cteliac. Between these, in 

 some fishes, the brachial arteries are sent off from the abdominal 

 aorta : these vessels in the large-finned Torpedos and Chima^rre have 

 a partial investment of muscular fibres, like secondary bulbs, to 

 accelerate the circulation througli tliem.* 



In the Porbeagle Shark {Lamna cornubica) the two ca3liac arteries 

 each split into a bundle of small arterioles, which interlace with 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 ramifications of those trunks in the 

 ordinary way to the stomach and intestines.f The arterial branches 

 to the spiral valve in the Fox Shark are remai'kable for the rich 

 bundles of twigs by which they distribute the blood to that produc- 

 tion. In the Mediterranean Tunnies (T/iT/nnus and Auxis) the 

 branches of the ca^liaco-mcsenteric 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 gasti'ic 

 artery, in a small body chiefly composed of the cellular beginnings of 

 the returning veins, forming the vaso-ganglion so constant in all 

 higher Vertebrates, and called the 'spleen' {^/ig. 61. n). It is not 

 present in the Lancelet ; and the gland-like bodies near the cardia in 

 the Cyclostomes, and near the pylorus in the Lepidosiren, which some 

 have called ' spleen,' are more like the recognised remnants of the 

 vitellicle in osseous fishes, where a true spleen is actually present. 

 The vein of the spleen always contributes to form the ' vena porta3 ; ' 

 but it is important to note that it is not essential to the formation of 

 that vessel. The absence of the spleen in fishes is concomitant with 

 the absence of the pancreas ; and the increased size and complexit)'- 

 of the spleen is associated in some fishes with a corresponding 

 development of the pancreas. Thus there is an accessory spleen in 

 the Sturgeon ; and the spleen is divided into numerous distinct 

 lobules in Lamna, Selache (see part of the organ in Jig. 64. «), and 

 some other highly organised Plagiostomes. In most osseous fishes 

 the spleen is appended by its vessels, and a meso-splenic fold of 



* Duvcrnoy, xc. f xxi. 1841, p. 99. pi. 5. 



