Snakes’ Physiology and Anatomy in Detail
Snakes are highly specialized reptiles. Like that of birds, their body structure seems to have been stripped to the essentials for their way of life. Snake skulls contain many fewer bones than do those of lizards, for example. Snakes have no trace of forelimbs, and only a few of the primitive snakes retain vestiges of the hind limbs.
The loss of nonessential bones and other structures has meant greater flexibility of the body. While many bones have disappeared, the muscles of the body wall have become differentiated into tiny groups with individual actions. The body muscles of a snake have become more specialized and more highly organized than those of any other animal.
The bones of the skull, except for those of the braincase itself, are very loosely connected to one another, and the jaw muscles have become specialized so that each element can move individually. Thus the entire snake is constructed as a creature of high mobility and flexibility. It has discarded the heavy armor that is characteristic of many lizards, and of reptiles in general, for a body form and structure that allow it maximum strength combined with great agility.
The entire body surface of a snake is covered with rows of horny scales that serve to keep the animal from desiccating under dry conditions. These scales lack any trace of the base of bone that is found in many lizards and crocodilians and is completely different from fish scales.
The scales are not placed in a random fashion but follow definite patterns that are characteristic of different groups of snakes. In all snakes, except those that are highly aquatic or highly specialized for burrowing, there is a single row of very broad scales, called ventral scutes, down the underside of the body. The underside of the tail typically has two rows (sometimes one) of subcaudal scutes. There is also usually a single or paired enlarged scale, the anal scute, that covers the vent. The head scales are so differentiated and so regular that they have been given individual names.
The arrangement of these different scales and their numbers are typical for individual species of snakes as well as for groups of snakes. Thus scale features are used in the classification of snakes and as identification keys.
The skeleton of a typical snake is made up of the skull and jaws with their associated teeth and a long flexible backbone made up vertebrae with the associated ribs. There is no sign of a pectoral girdle (for forelimbs) or a breastbone, and only a few snakes have vestiges of the pelvic girdle and the hind limbs. The hyoid is represented by a U-shaped or Y-shaped structure.
The snake’s skull, except for the feeding mechanism that is described below, is a rather solid structure. The braincase surrounds the brain much more completely than it does in lizards. This provides the brain with greater protection, probably a necessary feature due to the large prey animals that most snakes feed on. Besides protecting the brain, some of the front element of the skull, such as the prefrontal, prevomer, and nasal bones, also protect the important sense organs, including the organs of scent, the Jacobson’s organs, and the eyes.
The jaws and the bones associated with them make up the snake’s main feeding mechanism, which is very different from that of other animals. Although the jawbones and palate bones may be bound closely together in burrowing snakes to help solidify the head for use as a ram, the typical harmless snake or boa has jawbones that are very loosely articulated.
The construction of the lower jaw is a typical snake feature. Rather than being fused together at the “chin,” the two halves of the lower jaw are held together loosely by muscles and flexible ligaments. Thus the ends of the lower jaws can be forced well away from one another, allowing the snake to swallow a very large object.
In addition to the flexible outer ends of the lower jaws, snakes also have’ two additional points of movement between the jawbones and the skull. The upper jaws and the bones of the palate are also loosely articulated with one another and with the skull and may be moved independently. All of these specializations serve a single purpose—to allow the snake to swallow large prey animals in a situation where the jaws must be the hands for manipulating the food as well as the structures that convey it from the mouth to the esophagus.
Teeth and Fangs
The mouths of most snakes have a very large number of teeth. As in most animals the maxillary bones of the upper jaw and the dentary bones of the lower jaw bear teeth. In snakes, however, two bones in the palate (palatine and pterygoid bones) also bear teeth, and some pythons have additional teeth on the median (premaxillary) bone of the snout. A typical snake, then, has four rows of teeth in the upper jaw and two rows in the lower jaw. These teeth may total 80 or more.
Typically the teeth are long, conical, needle-like structures. They are much alike from the front to the rear of the jaw and from one kind of snake to another. Snakes use their teeth merely to hold prey and to aid in pulling it into the gullet. Therefore, the teeth are usually recurved—that is, they point backward and into the mouth.
A few snakes that are adapted to feed on certain kinds of animals have specialized teeth. For example, snakes (Sibytiophis) that feed on tough-skinned skinks have numerous blade-shaped teeth, and those that feed on crayfishes and other hard-shelled prey have teeth with broad-pointed tips that are not likely to be broken off.
Many snakes have some of the rear teeth of the upper jaw enlarged. The hognose snake (Heterodon), for example, seems to use such teeth to puncture the toads that fill themselves with air to avoid being swallowed. In some snakes, these enlarged teeth are grooved to act as fangs for the administration of venom to the prey animal.
The members of the cobra family (Elapidae) and the vipers (Viperidae) have true fangs. Fangs are teeth. They are like hypodermic needles, with two openings—one near their base connecting with the venom duct and one near the tip for the injection of venom into prey.
Backbone and Ribs
The backbone of the snake is made up of many vertebrae—frequently more than 300. Those vertebrae in the body area are all very much alike. Each has many processes, or projections, for the insertion of muscle slips and a paired process for the attachment of the ribs. A special set of projections on the front and rear of the vertebrae lock adjacent vertebrae together. This arrangement gives the backbone unusual flexibility without the attendant danger that the vertebrae might slip and injure the spinal cord that they enclose.
The ribs are rather simple, single-headed, curved types. One pair is borne by each of the body vertebrae. The ribs do not join below but have free ends. They vary in size but are structurally much alike. However, a few snakes, such as hognose snakes, are able to pull some of their ribs out so as to flatten the body, or, as in the cobras, to form a hood.
The internal structure of snakes is typical of that of vertebrate animals and is very much like the internal structure of a lizard, although it is modified for an extremely long and slender body.
The digestive system is similar to that of other animals with a well-defined and often long esophagus, a stomach, and small and large intestines. The large intestine opens into a common chamber, called the cloaca, as do the ducts from the excretory and reproductive systems. These systems all exit from the body through a common opening called the anus, or better, the cloacal aperture. The liver is large but elongate and divided into separate lobes to help it fit into the streamlined body cavity.
The respiratory system is peculiar in several respects. Most snakes have only a single lung— the right one—the other having disappeared in the development of a streamlined body. Some of the primitive snakes, such as the boas and pythons, retain a left lung, though it is always much smaller than the right. Another unusual feature of the respiratory system is that the glottis, the entrance to the respiratory tract, is found on the floor of the mouth. The glottis is surrounded by muscles that can move it far enough so that it actually can be projected from the mouth. This seems to be an adaptation for eating large prey animals. It allows the snake to take a breath even when the mouth is occupied with a large meal.
The nervous, excretory, and reproductive systems of the snake are similar to those of most other vertebrate animals. A snake has a brain and a spinal cord, together with various sense organs. The kidneys are long, dark red structures found along the body wall in the posterior end of the body. One is placed somewhat ahead of the other instead of being direct across from one another, still another feature of streamlining. This is also true of the ovaries in the female and of the testes in the male. Both excretory and reproductive systems empty into the cloaca.
Several of the sense organs that are important to snakes are the same as those of humans, while others are different. Snake bodies are sensitive to touch, just as ours are. Although some snakes with rough-keeled scales may not feel slight sensations of pressure on their bodies, many smooth-scaled species seem to have skin as delicate and sensitive as human skin.
In snakes that hunt during the day, the eyes are probably the most important sense organs. Racers, in particular, may be seen chasing frogs or meadow mice through open areas. The lizard-eating tree snakes also appear to depend heavily on sight to find prey. The boom-slang (Dispholidus typus), an African lizard-eating snake, has extraordinarily large eyes. Some of the Asian tree snakes (Ahaetulla species) have eyes with horizontally elliptical pupils that presumably improve their ability to see forward, and the similar African bird snake (Thelotomis kirtlandii) has “keyhole” shaped pupils that allow it to see with binocular vision and thus judge distances accurately.
Some snakes that are active during the day, such as racers, have eyes that are specialized for acute vision in bright light. The lens of the eye is tinted with yellow to protect the sensitive cells of the retina from too much light. Snakes do not have movable eyelids, so they cannot “squint” or close their eyes. Other snakes, such as vipers, have eyes that are specialized for night hunting. They have vertically elliptical pupils (“cat-eyes”) that can open wide, like barn doors, to let in the least traces of light, or they can close up into narrow slits when the light is bright. Most snakes, of course, are not specialized in either direction but have round pupiled eyes without colored lenses.
In all snakes, the eye—whatever its kind-seems to be mainly stimulated by movement. A black snake may chase a rapidly jumping frog across a meadow, but the snake loses sight of the frog if it stops and sits still. Although the frog might be perfectly visible to a human, the snake must then use some other sense organs to attempt to find it.
Smell and Jacobson’s Organ
Snakes appear to have a good sense of smell through their tiny nostrils. More important for hunting, however, is another sense that humans do not have. This sensory system involves both the tongue and sense organs known as Jacobson’s organs. The flicking tongue of the snake is used to pick up chemical particles from the ground or the air. Then the tiny tips of the forked tongue are inserted into pits in the palate that contain masses of sensitive cells, which serve to identify the particles. These masses of sensitive cells are the Jacobson’s organs. Such organs are also found in amphibians, but they are especially well developed in snakes. With this apparatus a snake can follow the trail of a prey animal whose scent would be too faint for a bloodhound. A dog’s nose might catch the chemical stimuli in the air and perhaps on solid objects, but only a snake can pick it up easily from sandy soil. When snakes use both their sense of smell and Jacobson’s organs they can follow even a cold trail without difficulty.
The hearing ability of snakes is still a matter of dispute. The anatomy of the middle ear with the main hearing bone, or columella, attached to the lower jaw suggests that snakes cannot hear airborne sounds. There is no eardrum or thin membrane to collect such airwaves, and the snake’s typical lack of reaction to loud sounds seems to indicate that it does not hear. Yet some experimenters have obtained electrical impulses from the auditory nerve of a snake when it was stimulated by certain tones. This would ordinarily be interpreted to mean that the snake “heard” these sounds. The overwhelming evidence on the other side is, however, very convincing, and more experimental work will have to be done before it is known whether snakes can hear. There is no question that they can sense vibrations such as footsteps through the ground.
Pit vipers (family Crotalidae) have an additional sense that is helpful to them in finding prey animals in the dark. The viper’s “pits” are highly sensitive cup-shaped structures located on each side of the face between the eye and the nostril. This pit is sensitive to temperatures different from those of the background. The snake can sense an object that is either above the background temperature or below it. These temperature-sensitive structures are so situated on the viper’s head that there is only a narrow overlap of the sensation of the two pits. This area of overlap—directly in front of the snake—enables the snake to face a prey animal directly and then to strike it accurately even in totally dark surroundings.
Some boas, such as the American tree boas (Corallus species) have temperature-sensitive pits on the plates that border the lips. Although these pits do not appear as complicated as those of the pit vipers, they seem to work in the same fashion, serving to inform these nocturnal snakes of the presence and direction of potential prey.