388 PEOT'. &. B. HOWES ON THE 



(*. e., and more especially, of the Mammalia). Nearly all previous 

 observers have agreed in regarding it either as a portion or the 

 whole of the large intestine. It is characteristic of that part of the 

 large intestine which, in the higher Vertebrata, lies immediately in 

 front of that related to the inferior mesenteric vessels, that its 

 nutrient arteries arise at a point remote from it anteriorly {cf. 

 ante, p. 384) ; exceptions occur (ex. Salamandra, PL I. fig. 5), but, 

 even in such, a wide interA^al is recognizable between the vessels 

 in question and those of the inferior mesenteric series (a.m.). 

 Either the posterior artery of this set (where more than tv/o exist) 

 or the most posterior branch thereof (where either one [^Bana, 

 most TeJeostet] or two [JRaia] exist) invariably supplies, in the 

 higher Ichthyopsida and Eeptilia *, the head of the large intes- 

 tine, with a more or less considerable portion of the adjacent 

 base of the ileum. That to which I here refer as the " head " of 

 the large intestine includes so much of that viscus as is not 

 supplied by the inferior mesenteric vessel or vessels (i. e. the 

 short caecum and that v-^hieh in the Mammalia becomes the 

 greater portion of the colon). 



On turning to the Plagiostome fishes, a considerable variation 

 becomes manifest (fig. 7, i. to v.) in the number and arrangement 

 of these vessels. There are never more than three present, as 

 ordinarily enumerated ; four are indicated in the accompanying 

 diagram (fig. 7, iii., iv., v.), but the terminal one of the series 

 is usually excluded from the category, having been likened by 

 Hyrtl, whose nomenclature has been hitherto everywhere adopted, 

 to the inferior (posterior) mesenteric. As such this vessel is 

 customarily described. Herein there lies a contradiction ; for if 

 it be true, as I claim to have shown (ante, p. 386), that the 

 inferior mesenteric artery of the higher Vertebrata, with which it 

 has been compared, is one of a series characterized, in all its 

 variations, by the fact that its members do not arise at a point 

 anteriorly remote from that portion of the gut which they supply, 

 this vessel must be removed from that category, and the term 

 " inferior mesenteric " will become inapplicable to it. Hyrtl's 

 determination was based upon an examination of the vessels of 



* Cf. Hoffmann, 14. p. 1574. The term " inferior (posterior) mesenteric " 

 has here been applied to that which clearly represents the vessel so named by 

 Hyrtl in the Elasmobranchs, those which represent the inferior mesenteries of 

 the Amniota and Amphibia being termed rectal arteries (cf. pi. cxxxv. fig. 1). 



