NERVOUS SYSTEM--SENSE ORGANS
The brain is small and of very oblong shape. It consists of smooth
cerebral hemispheres, small optic lobes, a still smaller cerebellum, and
long olfactory lobes; the pineal body is not accompanied by a parietal
organ. The spinal accessory cranial nerve is absent, and the sympathetic
system is but feebly developed.
The eyes have been noticed above (p. 12). When normally developed they
are susceptible of a slight movement under the transparent disc, quite
independent from the cornea, which covers them, and from which they are
separated by the so-called "lacrymal chamber." There are two lacrymal
glands, one in front and one behind; the lacrymal duct opens into the
posterior nares. A sclerotic bony ring is absent.
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.
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.
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