Snake Anatomy Basics

Key Points

  • Snakes have a long narrow body that can be divided into four quadrants
  • Major structures of the first quadrant consist of the head, esophagus, heart, and trachea.
  • The anterior, vascularized portion of the lung(s), as well as the liver and stomach are found within the second quadrant.
  • The third quadrant contains the gallbladder, spleen, pancreas (or splenopancreas), and gonads. Coursing between these structures is the small intestine and adjacent to them is the right lung.
  • The fourth quadrant contains the junction between the small and large intestine, the cecum (in boas and pythons), kidneys, cloaca, and hemipenes.

Introduction

Snakes are members of the class Reptilia, order Squamata, and suborder Serpentes. There are over 3,500 species of snakes in the world, however, for the most part, the anatomy of the snake is consistent across species.

Snakes have a long narrow body adapted for crawling and their internal anatomy has evolved to fit into a long narrow tube. It is possible to divide this tube into four quadrants (Fig 1). Although the sequence of organs is the same for all species, the relative position and size of the viscera can vary significantly between and within families.

snake quadrant

Figure 1. Comparative anatomy of a colubrid (kingsnake) (left) and a boid (common boa) (right). Positions of organs by percent of body length (nose to vent) are represented to the left. This diagram was created by Dr. Paul (Tripp) Stewart and adapted for use in the first two editions of the Exotic Animal Formulary. Reposted here with the permission of Dr. Stewart and Dr. Jim Carpenter, Formulary editor. Click the image above to access a PDF for download.

The quadrant system can be useful in developing a general understanding of organ location. This knowledge can be beneficial in diagnostics and treatment, such as identifying an area from which to make a surgical approach for a specific organ system (Fig 2).

Using the quadrant system, the location of this lump within the snake’s coelom could provide valuable diagnostic clues to the anatomic structure(s) involved

Figure 2. Using the quadrant system, the location of this lump within the snake’s coelom could provide valuable diagnostic clues to the anatomic structure(s) involved. Photo credit: Erica Mede, CVT. Click image to enlarge.

 

Visit LafeberVet’s A Necropsy Guide to Serpentes for gross images of normal anatomy. Also see Mader’s Reptile and Amphibian Medicine and Surgery:  Figure 55.2 of Radiography-Snakes illustrates the relative position of organs in four major serpent families.

 

Quadrant 1

The proximal quadrant of the snake generally contains the trachea, esophagus, parathyroid glands, thymus, thyroid, and the heart.

  • The distensible esophagus is dorsal to the trachea.
  • The glottis is a small opening caudal to the tongue. When the snake consumes large food items, the glottis is pushed to one side and the jaw is lowered to allow respiration to continue.
  • The trachea, which consists of incomplete cartilaginous rings, begins in the first quadrant and extends down into the second quadrant.
  • The thyroid gland is ventral to the trachea and cranial to the heart.
  • Parathyroid glands (PTG) are single or paired. If paired caudally, the PTG are between and often medial to the cranial or caudal lobes of the thymus.
  • The thymus gland is bilateral and consists of two lobes, which are usually asymmetrical.

Clinical Tip: The heart is typically located in the upper third of the body however its exact position varies greatly among species.

 

Quadrant 2

The second quadrant contains a continuation of the esophagus as well as the anterior, vascularized portion of the lung(s), and the liver.

  • The stomach is spindle shaped or filiform and clearly demarcated from the esophagus.
  • The liver is the largest organ within the coelomic cavity. This dark red to brown, cigar-shaped structure is pointed at both ends.
  • The left lung is vestigial or absent, except in boids (boas and pythons), where two lungs in boids are almost equal in length. The left lung is never greater than 85% of right lung size.
  • The liver and stomach are located approximately midway between the snout and the vent.

 

Quadrant 3

The third quadrant generally contains the stomach, gallbladder, spleen, pancreas, (or splenopancreas depending on the species), adrenal glands, gonads, and the lung(s)/air sacs. Coursing between these structures is the small intestine.

  • The gallbladder, a round, green to blue sac, is caudal to and distant from the liver and immediately cranial to the spleen and pancreas.
  • Boids have a separate spleen and pancreas, whereas other snakes have a fused splenopancreas.
  • The right lung extends just cranial to the right kidney. The posterior portion of the lungs is the non-respiratory, thin-walled air sac.
  • Gonads are paired and medial to the kidneys on either side of the vena cava. The left ovary is absent in several species from four families of snakes

 

 

Quadrant 4

The final quadrant contains the junction between the small and large intestine, the cecum, kidneys, cloaca, and hemipenes.

  • Most snakes do not have a cecum, however a small cecum is present at the proximal colon in boas and pythons.
  • The paired kidneys are located cranial to the cloaca. The kidneys are elongated and lobulated. The right kidney is longer and more cranial than the left kidney.
  • The cloaca is a common chamber through which feces, urinary wastes, and reproductive products are passed. Snakes lack a bladder, therefore the ureters empty directly into the portion of the cloaca that receives urinary waste, the urodeum.
  • The hemipenes and cloacal gland are caudal to the cloaca. The hemipenes are paired copulatory organs that sit within pouches at the ventral tail base of the male (Fig 3). The hemipenes are closely associated with the scent or musk glands, which are present in both males and females

    Everted hemipenes in a rattlesnake

    Figure 3. Everted hemipenes in a rattlesnake. Photo credit: Tess Thornston via Wikimedia Commons. Click image to enlarge.

 

 

Musculoskeletal anatomy 

The axial skeleton of the snake possesses many unique features:

  • The head shows numerous specialized characteristics. The skull is more delicately built than other reptiles and is characterized by its kinetic nature (Fig 4). The great mobility of the skull paired with the absence of a mandibular symphysis, allows the snake to swallow whole prey much larger than the larger than the head or the diameter of the body (Fig 5).
  • The vertebral column is comprised of anywhere from 180 to more than 400 vertebrae, all of similar shape. These vertebrae possess more articulating facets than seen in mammals, which allows snakes greater mobility. All vertebrae except the first two cervical bones bear mobile ribs (Fig 6).
  • The ventral aspect of each rib is attached by muscle to the ventral scales.
  • Pythons, some boas, (and small worm snakes) possess pelvic vestiges (see external spurs below).

 

Lateral view of the skull of a Burmese python (Python molurus), with visible kinetic joints.

Figure 4. Lateral view of the skull of a Burmese python (Python molurus), with visible kinetic joints labeled. Red = highly mobile (diarthrosis), green = slightly mobile (amphiarthrosis), blue = immobile (synarthrosis). Photo credit: Mokele via Wikimedia Commons.

 

Snake skull and vertebral column

Figure 5. Snake skull and vertebral column. Note the wide gape and great flexibility of the jaw. Click image to enlarge.

 

Snakes have between 180 to more than 400 vertebrae. All vertebrae except the first two cervical bones bear mobile ribs. There is no sternum. Instead the ventral aspect of each rib is attached by muscle to the ventral scales.

Figure 6. Snakes have between 180 to more than 400 vertebrae. All vertebrae, except the first two cervical bones, bear mobile ribs. There is no sternum. Instead the ventral aspect of each rib is attached by muscle to the ventral scales. Click image to enlarge.

 

External features

TEETH

All snake teeth, including fangs, are shed throughout life. The teeth are not rooted, but are instead attached to the surface of jaw bones.

Visit Understanding Reptile Dental Anatomy: Clinical Applications for a discussion of snake teeth.

 

SPECIAL SENSES

  • The tongue is long, cylindrical and deeply forked. The tongue sits in a sheath beneath the glottis and it plays no role in swallowing. Instead the serpentine tongue is used for olfaction together with the vomeronasal organ on the palate (Fig 7).
  • Vomeronasal or Jacobson’s organ plays an important role in olfaction.  This structure consists of a pair of sacs found rostral to the internal nares. The tongue brings minute air particles into direct contact with the vomeronasal organ (Fig 8).
  • There is no tympanic membrane or middle ear cavity, however, a single ossicle, the columella, extends from the inner ear to the quadrate bone. The presence of this ossicle implies that snakes primarily detect low-frequency sound waves conducted through the ground (1).
  • Heat-receptive pit organs are specialized infrared receptors on the head of certain snakes used to detect prey items. Pit organs are extremely sensitive and allow the snake to navigate and find food in complete darkness. They can respond to changes in temperature as small as 0.002°C. Pit vipers (Crotalidae) possess facial or maxillary pit organs on both sides of the head, between the eyes and external nares. These sensitive olfactory organs detect infrared heat. Labial pit organs are similar structures found in all pythons and some boas (Fig 9).
  • The eyeball is small and lacks a retractor bulbi muscle. There is no sclerotic ring. Snakes lack movable eyelids. Instead the cornea is protected by a transparent, vascular spectacle, which is an embryonic fusion of the two eyelids. There is an avascular retina.
The forked tongue is used in olfaction and plays no role in swallowing.

Figure 7. The forked tongue is used in olfaction and plays no role in swallowing. Photo credit: Erica Mede, CVT. Click image to enlarge.

he forked tongue delivers odor particles to the vomeronasal or Jacobson’s organ located in the roof of the mouth.

Figure 8. The forked tongue delivers odor particles to the vomeronasal or Jacobson’s organ located in the roof of the mouth. Image credit: Fred the Oyster via Wikimedia Commons. Click image to enlarge.

 

Heat-receptive labial pit organs (arrows) are found in all pythons and some boas.

Figure 9. Heat-receptive labial pit organs (arrows) are found in all pythons and some boas. Photo credit: Erica Mede, CVT. Click image to enlarge.

 

SCALES

  • Reptile skin is covered primarily by scales (Fig 10).
  • As the snake grows, the skin (including the spectacles) is shed periodically. This process, which is called ecydysis, is regulated by thyroid hormone. Skin is generally shed in one piece, although large snakes, exceeding 3 meters in length, may shed skin in pieces. The frequency of shedding will vary with the snake’s rate of growth but will typically occurs two to four times per year.

A snake will become anorectic and inactive as the time for shed approaches. Approximately 14 days prior to shed the snake will develop a dull, grayish appearance as lymphatic fluid fills the space between old and new epidermal layers. The spectacles will also have an opaque, blue color approximately 7-10 days prior to the shed and then clear 2-3 days before ecdysis occurs. Handling the snake during this time should be avoided to prevent damaging the underlying epidermis. Incomplete shed (dysecdysis) and/or retention of the spectacles are common clinical problems (Fig 11).

  • Spurs are the external component of the vestigial pelvic remnants found in some snakes, like boas and pythons. Spurs are short, sharp, keratin-covered structures. Well-developed musculature allows the spurs to move, which is important for male pythons during courtship and mating (Fig 12).
Close-up from the scales of the common garter snake (Thamnophis sirtalis).

Figure 10. Close-up from the scales of the common garter snake (Thamnophis sirtalis). All reptiles are covered with scales. Note the overlapping pattern and the presence of soft integument between the scales. Photo source: Newmansr via Wikimedia Commons. Click image to enlarge.

 

The normal spectacle or “eye cap” should be clear and smooth. A retained spectacle has wrinkles and/or opacity.

Figure 11. The normal spectacle or “eye cap” should be clear and smooth. A retained spectacle has wrinkles and/or opacity. Photo credit:  Erica Mede, CVT. Click image to enlarge.

 

External spurs may be used during courtship in boids.

Figure 12. External spurs may be used during courtship in boids. In this image, the tail is to the left and the head is to the right. Photo credit: Dr. Christal Pollock. Click image to enlarge.

 

 

References