Snake Anatomy & Physiology 1

2 Mar 2010   | SNAKES

This article by Jerry Cates, first published in January 1999, was last revised on 23 April 2016. © Bugsinthenews Vol. 01:01(02).

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Snake Anatomy Diagram

Snake Anatomy Diagram

 Details of General Anatomical Features (click on the diagram at left for an enlarged version):

Snakes have no moveable eyelids, limbs, ear openings, sternums, or urinary bladders. Most species have only one functioning lung, although many have a second, vestigial (essentially non-functioning, or only marginally functional) lung. The organs in the snake body are necessarily elongated, to fit within the narrow confines of its body cavity. Lizards differ from snakes anatomically by having their two lower jawbones fused together, while the lower jawbones of the snake are connected by a flexible band of tough tissue that enables the two bones to articulate separately. The quadrate bones that connect the lower jaw to the skull are, in the lizard, small and fixed, but in the snake long and flexible; this gives the lizard the ability to crush its prey with its jaw, a capability lacking in the snake (which is why the fangs of pit vipers are in the upper rather than the lower jaw), but allows the snake to expand its mouth to receive and swallow large prey, which the lizard cannot. Lizards have only a few ribs, while snakes have many.

Prey Preference & Snake Size

Each species of snake native to Texas has its skull, jaws, and teeth specially adapted to capturing, immobilizing, and ingesting the prey favored by that species.  The smallest snake found in Texas is a species of blind snake, in the family Leptotyphlopidae, that is no larger than an earthworm (and often mistaken for one).  The two species of blind snake found in Texas eat termites and ants.  Many other species of snakes found in Texas never grow much larger than 8-12 inches in length.  Thus, many of the small snakes we meet in the wild are adults, not merely juveniles of larger snakes. Many snake species spend their lives preying on the myriad worms, insects and anurans found in our landscape. Snakes continue to grow throughout their lives, and the largest specimens of a species are usually the oldest.  Of course, treat any snake as venomous until you can identify it precisely.  Juvenile vipers can be more dangerous than the adults of their species.

Skeleton

The skeleton of a snake, though sometimes described as a simple structure, is anything but.  A complicated–architecturally and functionally–skull at its head is followed, to the tail, by at least five divisions of vertebral structures.  First come two un-ribbed “Atlas” vertebrae, followed by 1-3  un-ribbed neck vertebrae. Next follow from 100-600 ribbed mid-body vertebrae and 2-10 un-ribbed pre-pelvic and pelvic vertebrae.  These are attached to a variable number of un-ribbed tail vertebrae that are often modified, individually and collectively, to perform specialized functions peculiar to the species.

Mid-body ribs–only the mid-body vertebrae have ribs–attach to and articulate with the mid-body vertebrae, and laterally, with each other.  This leaves their tips, that stretch to the snake’s belly region, unattached.  Such an arrangement gives the snake considerable ability to expand its girth to swallow prey and convey it down the gullet, to the stomach, into the small and large intestines, and finally to the anal cavity, from which residual, unmetabolized, waste matter is expelled.

Skin

A snake’s skin consists of a scaly integument that protects the animal from abrasion and prevents water loss.  The integument on the snake’s back and sides is thinner than that of the belly.  Scales on the back and sides are more numerous than belly scales and are either smooth or keeled with noticeable ridges.  Belly scales, also referred to as scutes, are thick and large, and are commonly arranged in narrow strips that extend from one side of the belly to the other; under-tail scales — those extending beyond the snake’s venter — are either singular like those of the belly, divided, or initially singular then divided; the particular arrangement of the under-tail scales is distinctive to a species.  Head scalation, too, is used to distinguish between species and individual scales or scale groups on the dorsal, lateral, ventral, and frontal head used in identification have special names.

Coloration and Markings

Scale coloration and marking is the most common method  used by the public to distinguish one snake species from another, but many species exhibit considerable variation in color and marking, even within litters, and — to complicate matters even more — the juveniles of some species are scarcely representative of their adult coloration and marking.

Markings on a snake’s body may be absent or may consist of an imprecise nomenclature of stripes, saddles, blotches, spots, diamonds, bars, hourglasses, or bands, each of which may also have nuances of coloration on their borders and interiors.  A representative sample of a few of the various colorations and markings found on Texas snakes can be reviewed by CLICKING HERE.

Combinations of these marks on some snakes produce a wide array of patterns that can be confusing to the neophyte herpetologist.  If, for example, you happen to see a snake with red, orange, or gold diamonds or triangles down its back,  you may actually be looking at a juvenile garter with a red, orange, or yellow spinal stripe, bordered by black spots that blend into each other. That arrangement, when observed at a glance, has the appearance of brightly colored diamonds or triangles.  It is wise, therefore, to invest some time looking at the various markings found on the snakes common to Texas, so that you will recognize the individual marks when you see them, rather than the confusing patterns that combinations of such marks sometimes produce.

Skin Shedding (Ecdysis)

Snakes shed their skins when they outgrow them; snakes with slow metabolisms eat less often, grow slowly, and shed infrequently; those that eat often have, in general, a more robust metabolism, which leads to faster growth and more frequent shedding.

Shedding, also known as ecdysis, is hormonally directed and is carried out in distinct phases. New skin is produced under the existing one; when the new skin is complete, it secretes a fluid that separates it from the old skin and causes the latter to take on a bluish hue (since the eye covering is a part of the snake’s skin, this area becomes opaque, making the snake temporarily blind); just before shedding the intervening fluid is reabsorbed and the old skin returns to its earlier coloration until the skin is shed, usually in a single piece, beginning at the head, as the snake literally turns its old skin inside out.  When you find a discarded snake skin, the outer surface is actually the portion that was, prior to shedding, the inner surface of the skin.

Although the markings of the snake are only slightly evident in most shed skins, those slight marks, plus the clearly evident scale pattern in the skin, may be used to identify the species of snake involved.

Head, Skull and Jaws

Snake skulls consist, in general, of a bony, inelastic brain-case that is connected with the rest of the skull by flexible ligaments that enable the snake to exercise an extraordinary range of jaw articulation. The lower jaw is attached to the posterior skull, providing for a large, wide mouth.  The right and left lateral portions of the lower jaw attach to each other at the lower snout by elastic cartilaginous tissue that enables them to move freely and independently.

Teeth & Fangs

Snake teeth follow three forms.  The teeth of most non-venomous snakes–such as boas, pythons, rat snakes, whip snakes, ground snakes, and lined snakes–consist of two rows of numerous small teeth on the upper jaw, and one row of similar teeth on the lower jaw; the teeth on both jaws curve toward the back of the mouth, as in the Texas rat snake:

Elaphe spp head with mouth forced open

Elaphe spp head with mouth forced open

In between the non-venomous snakes and the virulently venomous ones are snakes with grooved fangs such as the hog-nosed snakes; such snakes have one row of moderate-sized teeth on each side of the upper jaw and a pair of rear-facing fangs positioned deep in the mouth; the groove in these fangs allow venom, supplied by glands on the upper head, to flow into the body of the snake’s prey during capture and ingestion; the venom of such snakes functions, in general, more as an anesthetic than as a tissue digester.  Several species of snakes native to Texas have such fangs, but none pose a threat to man because their fangs, being positioned deep in their throats, are useless as defensive weapons.  For that reason, we classify such snakes native to Texas as non-venomous, which, as a practical matter, they are. Certain species found in other parts of the world have similar fangs that can be used in defense, and such snakes are capable of envenomating humans, sometimes with even fatal results.

The teeth of venomous vipers, adders, mambas and cobras include hollow fangs that inject tissue-digesting and/or neurotoxic venom, under pressure, via tubes attached to venom sacs positioned in the lateral posterior of the head; the fangs can be fixed and numerous, as in the elapids (cobras and coral snakes); the Texas coral snake shown below is grasping its prey’s neck (a rough earth snake) while slowly injecting venom through small, hollow fangs, into the prey’s body:

Texas coral snake killing rough earth snake

Texas coral snake killing rough earth snake

In pit vipers, such as copperheads, cottonmouths, and rattlesnakes, the hollow fangs consist of a pair of relatively long, erectile, and retractable teeth similar to hypodermic needles:

Organs of Respiration (Western Cottonmouth)

The snake’s respiratory system consists of a glottis at the front of the mouth that leads to a windpipe; the glottis is moveable to prevent it from being closed off during prey ingestion; although it may appear that the mouth is completely filled with prey when the snake is swallowing food, the moveable glottis, whose opening is on the floor of the lower jaw, remains open so the snake can continue to induct and exhaust air from its lungs and air sacs. The windpipe, extending from the glottis, divides into two bronchial tubes, each leading to lungs; the right lung is primary and the left is secondary, considerably smaller and either non-functional or only marginally so.  The forward portion of each lung is used for gas exchange and is highly vascular (supplied with blood vessels); the posterior portions of each lung are hydrostatic, non-vascular air sacs that, in the right lung, extends nearly to the venter and serves to regulate pressures inside the body.  Induction and exhaustion of air is performed, not by a diaphragm, but by the movement of the ribs under the control of specialized muscles.

Organs of Smell: the Organs of Jacobson

Coborn (1995) points out that all snakes have a mobile, two-pronged tongue that continuously flicks in and out through the labial aperture at the front of the mouth. The tips of the tongue collect scent particles from the air or from objects in the environment and convey them to a pair of highly developed vomeronasal (Latin: vomer = “plowshare” + nasus “the nose” = “nasal trenches,” a reference to structure) sensing organs, the Organs of Jacobson, in the roof of the snake’s mouth. These, in concert with the separate nasal organs of smell accessed through the snake’s nostrils, provide the snake with an extremely efficient sense of smell.

Organs of Respiration Western Cottonm

Organs of Respiration Western Cottonmouth

Organs of Reproduction and Copulation

In the female, the gonads are ovaries positioned in the same area of the body as the testes are in the male; ovaries are attached to oviducts that conduct eggs to the uterus (located between the ovaries and the anal cavity) where, in egg-bearing (oviparous) snakes, the eggs are fertilized by sperm from the anal cavity and then are encased in shells before being expelled.  In live-birthing (viviparous) snakes the eggs are fertilized in and continue to develop in the uterus until, at birth, the young are expelled through the anal cavity. Many snakes, e.g., within the Viperidae family,  are ovoviviparous, that is the eggs are fertilized in the ovaries, then encased in thin, leathery shells with yolks that provide all the nourishment needed during gestation; when the developing snakes are ready to hatch, they may do so within the mother’s body, and thereafter expelled through the anal cavity as with viviparous species, or the eggs may be laid intact a few hours or days prior to hatching.

The male produces sperm in testicular gonads, and the sperm is conducted from each teste to the anal cavity, where the male copulatory organ, the hemipenes, are positioned.  Hemipenes means “half-penis”; it was so named because early anatomists believed the two halves of the organ, each of which is a fully functional penis [in the reptilian sense, though not in mammalian terms], were drawn together during copulation.  In fact, only one of the paired hemipenes is used during copulation, although both are evaginated simultaneously.

The hemipenes are ornate, externally grooved organs that serve to conduct sperm from the anal cavity of the male to that of the female.  One of the evaginated hemipenes is inserted into the female, where it becomes enlarged to the point that it cannot be withdrawn until copulation is complete.  The external architecture of the hemipenes is such that it functions to transfer sperm from the male to the female without the use of internal ducts; the transfer is performed entirely via the organ’s external grooves.

Snake Taxonomy

Taxonomically, snakes are vertebrates, with a segmented backbone containing a plurality of vertebrae. They are cold-blooded and have scaly, dry skin, which put them in the same class as the crocodiles, lizards and turtles (Reptilia). Lizards and snakes share the same taxonomic order (Squamata), but snakes occupy a unique suborder (Serpentes), containing as many as 15 families (some authorities say 13 or 14). Four families of snakes are native to Texas: (1) Leptotyphlopidae, the blind snakes, (2) Colubridae, the largest family of snakes, includes rat snakes, whip snakes, garters, hog-nosed, king, patch-nosed, and milk snakes, (3) Elapidae, containing only one Texas snake (Texas coral snake), and (4) Viperidae, containing the vipers, including the true vipers (in the subfamily Viperinae) that lack pit organs, and the pit vipers (in the subfamily Crotalinae) that have such organs; in North America all our vipers are pit vipers, including the copperheads, the western cottonmouth, and all the rattlesnakes.

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— Questions? Corrections? Comments? Feel free to e-mail jerry.cates@bugsinthenews.info. You may also leave a comment in the space provided below.



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One comment on “Snake Anatomy & Physiology”

  1. Roy Gaddis May 3,2010 6:06 pm

    Hired to articulate a python skeleton.Recieved a box of ribs and vertebrae.Couldn’t find any text on python skeletal anatomy.FINALLY came across your site.Told me everything I needed.THANK YOU!

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