marked at the point where it stops, then removed and measured against the subcaudal scales. The scalation count determines whether the snake is a male or female, as the hemipenes of a male will probe to a different depth (usually longer) than the cloaca of a female. Skeleton The skeleton of most snakes consists solely of the skull, hyoid, vertebral column, and ribs, though henophidian snakes retain vestiges of the pelvis and rear limbs. The skull consists of a solid and complete neurocranium, to which many of the other bones are only loosely attached, particularly the highly mobile jaw bones, which facilitate manipulation and ingestion of large prey items. The left and right sides of the lower jaw are joined only by a flexible ligament at the anterior tips, allowing them to separate widely, and the posterior end of the lower jaw bones articulate with a quadrate bone, allowing further mobility. The mandible and quadrate bones can pick up ground-borne vibrations; because the sides of the lower jaw can move independently of one another, a snake resting its jaw on a surface has sensitive stereo auditory perception, used for detecting the position of prey. The jaw–quadrate–stapes pathway is capable of detecting vibrations on the angstrom scale, despite the absence of an outer ear and the lack of an impedance matching mechanism—provided by the ossicles in other vertebrates—for receiving vibrations from the air. The hyoid is a small bone located posterior and ventral to the skull, in the 'neck' region, which serves as an attachment for the muscles of the snake's tongue, as it does in all other tetrapods. The vertebral column consists of between 200 and 400 vertebrae, or sometimes more. The body vertebrae each have two ribs articulating with them. The tail vertebrae are comparatively few in number (often less than 20% of the total) and lack ribs. The vertebrae have projections that allow for strong muscle attachment, enabling locomotion without limbs. Caudal autotomy (self-amputation of the tail), a feature found in some lizards, is absent in most snakes. In the rare cases where it does exist in snakes, caudal autotomy is intervertebral (meaning the separation of adjacent vertebrae), unlike that in lizards, which is intravertebral, i.e. the break happens along a predefined fracture plane present on a vertebra. In some snakes, most notably boas and pythons, there are vestiges of the hindlimbs in the form of a pair