Connection between the Capillary Bed and the Larger Vessels of the Lungs of Some Reptiles

There are fundamental differences in the topography of the pulmonary capillary bed in reptiles compared with amphibians. The microscopic differences can be related to macroscopic differences in the topography of the larger vessels. In amphibians, the large arteries remain as far as possible under the pleura, and the large veins are situated toward the interior of the lung; this is central differentiation. In reptiles, all large vessels '!;re under the pleura; here there is subpleural differentiation. The arteries enter the parenchyma from the pleural side, as in amphibians. Small arteries, arterioles, and precapillaries are surrounded by a space, optically vacant. This space consists of periarterial and lymph spaces which diminish and disappear as the arteries approach the capillaries. The precapillaries turn gradually into capillaries without marked distinction. The postcapillaries are short and branched. They are much more meandering and of varying caliber. But there is a fundamental difference from the amphibians, for in reptiles the postcapillaries do not tend toward the interior, but, like the precapillaries, keep to the surface of the lung. So it is in all complicated regions of the lung with respiratory capillaries. In the terminal thin-walled sac of snakes, the capillarization is only nutritive or there are no capillaries at all. In the lungs of amphibians, there is only a respiratory capillarization. The differences in venous topography indicate a fundamental differ­ ence in structure between amphibian and reptile lung. Further, although there is no real difference as regards small vein and precapillary topogra­ phy, there are basic differences in the distribution of capillaries.