The air enters their lungs chiefly in a direct course from the nostrils, only by the mouth when open. If you observe the flatness of the head, and the very short space that can exist between the nose and the mouth of snakes, you will readily trace the communication between the entrance to the trachea and the outer air through the nostrils when the glottis is not closed. Professor Owen, in his _Anatomy of the Vertebrates_, vol. i. p. 528, describes this process fully. In the foregoing description I have borrowed from him, as well as from Dr. Carpenter, Todd, and others; but as there is nothing like ‘seeing for oneself,’ I would persuade my readers to watch a snake for a few minutes. An inspiration at intervals will be easily discerned by the expansion of the body. You will also perceive partial or slighter breathings, and the trunk dilating and expanding gently through a sort of internal respiration which is going on; every now and then comes the deeper, fuller breath. You may perceive that sometimes one short portion of the body expands, as if the lung in that part only were at work. This is more easily seen in the larger snakes. I have watched these for a quarter of an hour or more at a time, during which period only a comparatively short portion of the body showed any signs of breathing. Schlegel, who carefully studied this action, observed sometimes as many as thirty such partial dilatations of the trunk and lung between two full inspirations. In the large reticulated python I once saw that about two feet of the body, viz. four to six feet from the head, dilated with occasional and irregular inspirations, and no other part. By and by slight indications of breathing were observable much lower down, many feet apart from the previous action, while during the whole time I was watching I saw not one full and entire inflation of the lungs. This was on a rather chilly September afternoon, and the python had partaken of a couple of ducks for dinner the previous day, and it was a time when inactivity is usual. In a rattlesnake, on the same day, similar partial and irregular respirations were observable, this serpent having caused four rats to disappear at his last night’s supper. Sometimes you can discern no indication whatever of breathing for a very long time. When the reptiles are not in health, when they are about to cast their skin, or when in a half-torpid condition, you may observe this. When a snake yawns—a long and leisurely proceeding—the lungs are doubtless greatly refreshed; otherwise these reptiles do not rest with their mouths open, and the only possible access of outer air by the lips being through the chink appropriated to the service of the tongue (and which is as exactly opposite the opening of the tongue-sheath as the nostrils are opposite to the glottis), they must breathe almost entirely through the nose, _except when yawning_.