Monday, January 16, 2012
Snake lack limbs, a sternum (breastbone), a shoulder girdle (skeletal elements for support of the front limbs), external ear openings, and a urinary bladder. In addition, the pelvic girdle (skeletal elements for support of the hind limbs) is absent in most snakes.
Snake also lack internal mechanisms for maintaining body heat and therefore have variable body temperatures (poikilothermy). Body temperature may be controlled behaviorally, however, by moving in and out of the Sun's rays. Metabolism varies greatly with body temperature. Temperature zone snakes are subjected to potentially fatal low winter temperatures. Such snakes hibernate, finding shelter in deep underground burrows or crevices, where they remain inactive and their metabolism slows dramatically. Conversely, some snakes may estate in deep, cool, subterranean receives during exclusively hot and dry periods.
Partly as an adaptation to permit the swallowing of large food (item whole, the snake's skull has been increasingly advanced snakes the only solid portion of the skull is the bran case, the upper skull consisting broadly of the brain case in the rear; small protective bones around the eyes and snout in front; and four rather long tooth bearing bones below. All four tooth bearing bones are loosely attached to the skull by elastic ligaments and are independently movable. The bones of the snout are also movable and allow the snake to bend its snout upward.
Except for the blind snakes, families Typhopodae, Anomalepidae, and Leptotyphopidae, whose rudimentary eyes are situated beneath the head scales, a snake's eyes are permanently covered and protected by the transparent lower eyelid. The lower eyelid is fused to the vestigial upper eyelid, snakes, therefore, cannot blink or "close" their eyes. The lenses of snake's eyes are rounded and are normally adjusted for distance vision. To focus on near objects, the snake moves the lenses forward. Although the eyes are normally set for distance, a snake's distance vision is typically poor except for detecting movement.
Snakes lack external ear openings, eardrums, and middle ear cavities. The small, sound conducting columella bone, however, is present. When the snake's head is on the ground, earth borne vibrations (sounds) are picked up by the lower jaw and skull and are transmitted by bone conduction to the columella and through the columella to the inner ear. Certain experiments have shown that the snake's inner ear, as well as specialized nerve endings in the skin, can detect low frequency airborne vibrations.
The Jacobson's or vomeronasal, organ is located in front of the roof of the mouth. It is a chemical receptor capable of detecting tinny chemical particles, including those given off in the body scent of animals. The snake's forked tongue is flicked out to allow the chemical particles in the air to adhere to or dissolve in the moisture on the tongue. The tongue is then brought back to the Jacobson's organ, where its forked tip inserted into the organ's two opening and the chemical particles identified.
Snakes can detect heat, or infrared rays through specialized groups of nerve endings scattered through the skin. These heat receptors are presumably involved in sun basking and other thermoregulatory behaviors. More specialized heat receptors are present as pits along the upper lip margins of many pythons and boas. In the pit vipers, such as the rattlesnakes, the specialized heat receptors (here called pit organs) are highly developed and occur as a single pair at the front of the head. The paired frontal placement of the pit organs enables the rattlesnake not only to detect the presence of warm bodied prey in the dark but also to strike at it accurately up to a distance of about 50 cm.