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. The heart in most cases is situated between the anterior seventh and the anterior fourth of the body; it may be much farther back, beyond the anterior third, in _Doliophis_, _Platurus_, and some Viperidæ and Amblycephalidæ, in the middle in _Chersydrus_. It is of rather elongate form, enclosed in a pericardium in which it lies freely, and has a sinus venosus, two auricles, and a single ventricle divided by a septum. Three arteries leave the ventricle, the pulmonary and two systemic arches. The right systemic arch gives off the carotid artery, which in many snakes, the common Grass-snake for instance, may branch into two, or in others be double from its origin. The anterior abdominal vein is single in most snakes, double in some Boidæ, and conveys blood from the ventral body-wall to the liver. The caudal vein is continued as the renal portal. Veins which have been regarded as remains of the two posterior cardinal of lower Vertebrates have been found in some of the Boidæ. The bifurcate transverse processes of the vertebræ at the limit between the body and tail enclose the lymph-hearts, which are large and more or less elongate, metamerically divided into several chambers, the right often more developed than the left. The thymus gland lies on each side of the trachea, near the heart, and the thyroid gland is in the middle line, close to the base of the carotid artery.