384 PEOF. G. B. HOWES ON THE 



two sets of trunks — an anterior set (the so-called inferior 

 Qjosterior] mesenteric) (a.sm., fig. 1), arising in advance of the 

 gut and passing obliquely backwards ; and a posterior set, arising 

 in the immediate vicinity of the gut and passing directly down- 

 wards. The alimentary tract of the higher Vertebrata receives 

 its arterial supply from two sets of vessels — an anterior set (the 

 coeliac and superior mesenteric arteries), which arise close 

 together or from a common trunk ; and a posterior (the inferior 

 mesenteric artery or arteries), which arise far back and pass 

 either directly downwards or but little obliquely backwards or 

 forwards. These well-known facts at once suggest homology 

 between the smaller vessels which I have herein described for 

 the Skate (a.im. of fig. 1) and those of the inferior mesenteric 

 series as ordinarily understood. The lack of oblique disposition 

 is not, however, the only distinctive feature of these vessels. 

 Those arteries known in the higher Vertebrata as the inferior 

 mesenteric are so called from at least an analogy to the single 

 vessel to which the term was first applied in mammals *. The 

 inferior mesenteric artery of the Mammal supplies the rectum and 

 the posterior section of the colon alone — it supplies, that is to 

 say, the posterior portion of the large intestine ; be this viscus 

 relatively short (as in the Cat and Dog), or relatively long (as 

 in the herbivorous mammals), the trunk of the inferior mesenteric 

 artery never undergoes marked displacement either in a back- 

 ward or forward direction. Such adaptive change as this vessel 

 exhibits is of the nature of an extension of its brauches along the 

 dorsal wall of the gut, without displacement of its main trunk. 



That the above remarks apply equally to most Amniota can be 

 readily proved on appeal to the works of leading authorities. 

 Among the Ichthyopsida, however, the recognition of a similar 

 condition of the arterial supply has, to a great extent, escaped 

 notice. To take a leading example, i. e. the Common Frog. 

 In that animal, as is well known, the alimentary canal receives 

 its arterial blood for the most part from a single coeliaco- 

 mesenteric trunk ; this supplies that viscus with its appendages 

 from the post-oesophageal region to the middle of the large 

 intestine (c/. "Wiedersheim 8. Abth. ii, pp. 76-77). There 

 arises from the aorta, almost immediately in front of its point 



* It is said to be absent in the Didelpkia (Owen, 25. p. 541) and Monotre- 

 mata (Owen, 24. p. 391) ; Hyrtl denies this (17. p. 7) for Ornithorhynchus. 



