The olfactory organ proper is little developed, but is accompanied by an accessory organ, Jacobson's organ, consisting of a pair of pediculate, cup-shaped sacs, between the nasal sacs and the roof of the mouth, encapsuled by the vomers and the turbinal bones, lined by olfactory epithelium, and opening in the mouth just in front of the choanæ. As this organ, richly provided with nerves, communicates with the inside of the mouth, its function may be to smell the prey as it passes through previous to deglutition. Snakes cannot be credited with a keen sense of smell, although undoubtedly guided by it during the nuptial period. In the more thoroughly aquatic snakes, the nostril may be closed, when respiration is suspended, by a spongy tissue, which acts as a stopper, and such nostrils are called "valvular," although a valve is not, in the strict sense, present; when the animal breathes, the nostril is opened by a compression, through special muscles, of the cavernous tissue. In some Sand-snakes the narial opening may be reduced to a crescentic slit. 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 The tongue is entirely retractile into a sheath below the glottis and opening in front of it; it is always withdrawn into the sheath when the snake bites or feeds.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.