Because these scales slide under each other by a kind of inclusion, so as to permit the ventral surface to shorten or lengthen at the will of the animal. When some of the foremost scales are pressed on the ground, those behind are brought forward, and in their turn, supporting the body, enable the forepart to advance. To qualify the scales to do this with greater advantage, they are connected with one another, by means of muscular threads and a longitudinal band, and are likewise aided by the peculiar mechanism of the ribs, which last are connected with the ventral scales by a flexible cartilage. The body, in general, is of a rounded form, but, when preparing for progressive motion, the ribs are drawn so as to flatten the scales of the belly, and by moving anteriorly or posteriorly, give to the scales with which they are connected, a corresponding degree of motion. The ribs in this case act as limbs to the scales, which may be compared to feet. This singular use of the ribs of snakes, in assisting progressive motion, was detected by the acute Tyson, and has been still further illustrated by Sir Everard Home. - Fleming.
Sir Everard Home was led to this discovery of the aid afforded by the ribs, to the whole tribe of snakes, in the progressive motion of those animals, by the following circumstances. A snake of unusual size, brought to London to be exhibited, was shewn to Sir Joseph Bankes; the animal was lively, and moved along the carpet briskly; while it was doing so, Sir Joseph thought he saw the ribs move forward in succession, like the ribs of a caterpillar. The fact was readily established, and Sir Everard felt the ribs with his fingers, as they were brought forward: when a hand was laid flat under the snake, the ends of the ribs were distinctly felt upon the palm, as the animal passed over it. This was an interesting discovery, as it tended to demonstrate a new species of progressive motion, and one widely different from those already known. - Notes to Blumenbach.
Of all animals, serpents possess by far the greatest number of ribs; which amount, in some, to 250 pairs.
Because they have no glands to supply the skin with moisture from within, but receive it by coming in contact with moist substances: it is possible, therefore, that the bags in the snake may be supplied in that manner, and the more so, as the cuticular lining appears perfect. Another peculiarity is remarkable in snakes so furnished; namely, an oval cavity, situated between the bag and the eye, the opening into which is within the inner angle of the eyelid, and directed towards the cornea, (or transparent membrane to protect the anterior surface of the eye.) In this opening there are two rows of projections, which appear to form an orifice capable of dilatation and contraction. From the situation of these oval cavities, they must be considered as reservoirs for a fluid, which is occasionally to be spread over the cornea; and they may be filled by the falling of the dew, or the moisture shaken from the grass through which the snake passes. - Sir E. Home.