OCR Text |
Show 368 MR. F. E. BEDDARD ON THE [Feb. 16, SOME GENERAL CONCLUSIONS. The facts detailed in the foregoing pages permit of some general conclusions as to the structure of the arterial system in the Ophidia. In this general survey of that system I have naturally compared my results with the most complete recent comprehensive survey of the vascular system in those Reptiles-that which is given in Bronn's ' Thier-Reich.' I am unable from my own observations to substantiate all that is said in that work, though in many points I can add nothing to the general statements therein contained. In the first place, though this conclusion can hardly be considered a novelty, it is important to note the prevalence of longitudinal systems which are developed in nearly all the organs of the body. The various regions of the alimentary tract possess longitudinally running vessels which are filled by several branches of the aorta. The same is true of the liver, the fat-body, the gonads, etc. This state of affairs has not been sufficiently fully emphasised in such figures of the arterial system as have been published. The anterior vertebral artery enters the thickness of the parietes at a variable distance from the head, varying-that is to say-according to the species, and the number of intercostal branches which it gives off upon its course is not, as might be inferred from the general account given in Bronn, three or four only. These numbers do indeed occur; but they are often considerably increased. Moreover, these arteries are, as has correctly been stated, always single-never paired ; and it may be added that they always perforate the body-wall exactly in the middle line, which is sometimes, but not always, the case with the intercostal branches of the common aorta. With regard to the latter, it will have been noted that they present many variations. In the first place, they are more or less numerous, and there is no exact correspondence between their numbers and the length of the snake. They may be single or arranged in pairs ; both arrangements occur in the same snake. When single they may enter the parietes exactly in the middle line, as is the case with the intercostal branches of the anterior vertebral artery; or they may lie to one side or to the other, in which case there is an alternation in position which is now regular now irregular. The gastric arteries vary rather more widely than might be inferred from the summary given in Bronn's ' Thier-Reich.' There are as few as one in Lachesis gramineus, three in Naia tripudians, and as many as ten or eleven in the genus Coluber. Their alternate arrangement to the right and left sides of the stomach is a noteworthy point. The hepatic arteries are sometimes more than ten to twelve in |