OCR Text |
Show 1895.] LUNGS OP SNAKES, AMPHISBJENIDJE, ETC. 697 lung, the embryological evidence, judging by the forms I have been able to study, is not so clear, because I have found no trace of more than one lung from the first. Thus in Vipera aspis and Typhlops lumbricalis I have stages which show the lung from an early stage inclining to the right side, after the manner of the right lung in the corresponding stages of such a form as Tropidonotus natrix, but there is no trace of a left lung. In the case of many morphological questions, it is considered sufficient to study the development of a single typical species. But in the present case this is not so ; for mygainsayers represent that in some Snakes one lung is developed, and in some the other. N o w embryological evidence is of course the most convincing, but it is manifestly hopeless to think of studying the development of the lungs of every species of Snake, and, in the absence of embryological evidence, that of comparative anatomy is quite cogent enough I think for our present purpose. I therefore propose to show how w e may easily tell the right lung from the left in any grown Snake by the light of comparative anatomy. In most pulmonate vertebrates there can of course be no doubt as to which is the right and which the left lung, for the two lungs hang in separate lateral portions of the body-cavity, separated from each other by one or, more usually, by two membranous septa. There can be no question about the matter in the case of Amphis-baenians and other Lizards, and any discordant statements about the lungs^of these animals must be simply the result of a mistake, whether on the part of the observer, the compiler, or the printer. With Snakes, however, it is otherwise. In Snakes as we know1 the body-cavity is in its anterior region obliterated except for the pericardium and the two sacs which encase the right and left halves of the liver; and moreover the viscera show a displacement of a more or less rotatory character. It thus happens that though, in the great majority of cases, the rudimentary lung, if present, will be found just where, after seeing the rudimentary lungs of snake-like lizards and of Gymnophiona, and also on embryological grounds, we should expect to find the rudiment of the left lung of a Snake-[viz. on the left posterior border of the heart]-still there are a few species, e. g. Heterodon platyrhinus [see (7) pi. xv. or (8) pi. xxviii., and figs. 1-4 of this paper], in which first appearances are somewhat deceptive, so far as the rudimentary lung is concerned. In like manner, though the larger, more dorsally situated lung which Cope speaks of as the " left lung " has in most cases, to myself personally, appeared pretty clearly to be the right lung, still in many cases the position of this lung is so far median, or partly inclining to the right and partly to the left side of the animal, that an observer whose studies had not led him to investigate closely the relations of the organs in these animals might be in doubt. Yet once looked at the right way, the lungs of Snakes present hardly more difficulty than the lungs of Lizards and Amphibians. 1 Cf. Proc. Zool. Soc. 1892, pp. 477-498. |