Internal anatomy of the snake
"Arms and legs gone, no ears, only one functional lung, voiceless, eyelids missing...a human being under these conditions would be institutionalized and under constant care..." James A. Peters, Encyclopedia Britannica 15th edition
The internal anatomy of snakes shows their obvious relation to other vertebrates; their organs, tissues, and germ cell layers are all very similar to those of other scaled reptiles (order Squamata), and even to other vertebral species (subphylum Vertebrata).
However, their differences from other vertebrates are even more distinct than their similarities. The body of a snake is long and elongated, like a tube, and certain adaptations have been made along the evolutionary chain in order to fit their organs into this form.
Other adaptations have also been developed in the evolutionary history of the suborder Serpentes, with the result of this highly specialized carnivore. Here are a few of those adaptations:
Lungs: All snakes are essentially one-lunged. Their left lung is usually vestigial, sometimes completely absent, and their right lung is enlarged and elongated, and has much less cartilage in it than other vertebrates. In aquatic snakes, the left lung's anterior portion still functions, albeit not for gas exchange. It works as buoyancy organ during swimming.
Jaws: The lower jaw of snakes is loosely attached, with ligaments connecting the anterior left and right halves of the mandible. The left and right halves are generally also connected with a relatively loose ligament, allowing separation and movement of both halves. When the snake ingests a large meal, the jaw easily pops out of its hinge, to allow food to enter the esophagus. After swallowing its prey, the snake will "yawn" widely, and snap its mandible back into place.
Spine: Snakes generally have between 200 and 400 vertebrae. The "tail" vertebrae usually make up less than 20% of the total, and are the only vertebrae without ribs attached. The ribs and vertebral column of the snake provide solid anchoring points for the strong muscles required for limbless locomotion, and are necessary much farther down the torso than in other vertebrates.
Skin: It's not slimy, for one! Despite some snakes looking like they have a sheen to their scales, no snakes secrete "slime" or mucous to coat their skin. Only amphibians and worm-type creatures do that. Snake skin is incredibly flexible, to accommodate the large meals that are consumed, and is comprised of scales, which are a protective extension of the epidermis. Scales also allow snakes to grip the ground or trees they're climbing. Snake eyes are covered in clear scales, allowing them to be protected without eyelids.
Ears: Obviously, snakes have no external ears. However, they still have inner ears. When soundwaves hit their skin, the vibration is transferred through the muscle and bone, and into the inner ear, where it's processed. Though the ability to sense directional vibration in snakes is generally highly developed, the sense of "hearing" as humans know it is relatively poor.
Sight: This is one trait that varies widely between snake species. Some are nearly blind, sensing only light and dark, while some can spot prey from far away. No snakes can see in color, but some snakes (the pythons, pit vipers, and some boas) can see infrared images - that is, they can sense the heat radiating from warm-blooded animals, allowing them to hunt prey at night.
Tongues: Snakes do not have a sense of taste, in the way that humans think of "taste". Instead, their tongues "test" the air for certain compounds, bringing the air particles back into their mouth, into their vomeronasal (Jacobson's) organ, which can tell if there are predators or prey in the area. Some snakes that live in aquatic environments, such as sea kraits and boas, can also use this sense underwater.
All images: Brehms Tierleben, Allgemeine Kunde des Tierreichs. Dr. Otto zur Strassen, 1913.
