The sense of hearing is not much developed. Tympanum, tympanic cavity, and Eustachian tubes are absent. In the typical snakes a long columellar rod (the stapes), with a fibrous or cartilaginous pad at the outer end, extends from the fenestra ovalis in the cranium to the quadrate, but in the degraded burrowing forms the stapes is a small bony plate closing the fenestra ovalis.
With one exception (Eryx jaculus, which is said by Schreiber to lap like a lizard), the tongue is not used for drinking or for the prehension or gustation of food, nor for hissing, but is a tactile organ protruded on any object the snake wishes to probe. It is slender and deeply bifid at the end, smooth, very protractile, often quite to the length of the head, and furnished with many sensory corpuscules. It is darted and vibrated on the least excitement, and is usually looked upon by the ignorant as a “sting.” In most snakes it is much pigmented, dark brown or black; in a few it is flesh-coloured or bright red. The tongue is entirely retractile into a sheath below the glottis and opening in front of it; it is always withdrawn into the sheath when the snake bites or feeds.
Other organs, which, in the absence of a satisfactory explanation of their use, have been termed “organs of a sixth sense,” reside in the head-shields and scales of many snakes, and in the deep pits on the sides of the head which are characteristic of various Boidæ and a few Colubridæ.
Scales often show, near their posterior extremity, one or two small light spots or impressions, caused by a thinning of the epidermis, which have been called “apical pits”; they appear to coincide with the terminations of nerve fibres extending along the epidermal folds of the skin. Similar organs sometimes form series on the borders of some of the head-shields, this being particularly noticeable in the Typhlopidæ.
The large and deep pit situated between the nostril and the eye (loreal pit) in the Crotaline Viperidæ—whence the name Pit-vipers, or that of “cuatro naricas” which is bestowed on them by the Spaniards of Mexico—is divided into two chambers: an outer with large external orifice, and an inner, rather more posterior in position and occupying an excavation on the outer face of the maxillary bone. The inner walls of these chambers are very thin and membranous, and form a partition separating the two, except for the presence of a minute opening; this partition is stretched across the hollow of the maxillary bone like the membrane of a drum, and is supplied with blood-vessels and nerves, the latter terminating in cells of variable form. The use of the organ, thus situated at the base of the poison fang, and therefore in close proximity to the sphincter of the poison duct, is still unknown.
Several of the Boidæ, such as Python and Corallus, have deep pits in some of the upper and lower labial shields, or also on each side of the rostral shield; these problematic organs are in all probability also sensory.
CHAPTER VIII
VISCERA
In most snakes there is a very marked asymmetry of the viscera and their blood-supply, the organs of the right side being anterior to, as well as larger than, those of the left.