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Basic Animal Anatomyby Erica Liszewski
The most important thing to recognize about animal anatomy is that all mammals have the same basic skeletal structure. A human has the same basic skeletal structure as an elephant. Birds, reptiles, and amphibians also have a very similar skeletal structure. All have a central spine, with the pelvis on one end and the skull on the other. The ribs spring from the spine to enclose the internal organs. The rear limbs attach the pelvis, and always have the same major joints. The forward limbs are often not directly attached to the rest of the skeleton and, again, share the same major joints. The differences in animals come from various bones being shaped differently to meet the needs of that particular animal. In this article I'll be focusing primarily on quadrupeds, but keep in mind that all animals share the same basic structure.
In this article I'll be focusing my diagrams on the dog and horse skeleton. I wanted to provide both a carnivore (dog) and an herbivore (horse) in order to highlight some of the differences between them. The dog and horse are probably the most well-studied of animals, so there is a lot of information available on both. I am also the most familiar with dog and horse anatomy, having drawn, studied, owned, and worked with both for many years. I'll start with the whole skeleton, and then look at each part in detail.
The dog is a carnivore, and while its evolution is not as specialized than the horse, the dog is constructed around teeth. The teeth of a dog are its primary source of food and defense. The dog has long thin limbs for running, although they don't need to travel as far or as fast as the horse. Dogs have four "weight-bearing" digits per foot, with a shorter dew claw on the front and sometimes back feet. It is interesting to note that all domestic dogs have the same anatomy, the differences between breeds are in the size and shape of the bones.
The horse is an herbivore, and its evolution has been driven by flight. A horse's life depends primarily on its ability to outrun danger. The horse has a shorter humerus and femur (upper leg), and a longer cannon bone (lower leg). This allows the horse to run faster, as it puts the muscle mass, and thus more of the animal's weight, closer to the body. The horse has only a single digit on each toe, although there are remnants of additional digits. Although the horse is more specialized than the dog, its structure is somewhat simpler.
Quadrupeds walk on all four limbs, and their bodies are built differently than bipedal animals. The spine of a quadruped contains the same types of vertebra as a human: cervical vertebra make up the neck, thoracic vertebra connect to ribs to form the chest, lumbar vertebra form the lower back, the sacrum attaches to the pelvis, and caudal vertebra form the tail. The exact number of each type of vertebrae varies somewhat between animals, but differences in shape are usually due more to the shape/size of vertebrae rather than a vast difference in number. Cervical vertebra are very flexible, and the first two are differently shaped to allow the head to pivot on the spine. Thoracic vertebra are much less flexible as each thoracic vertebrae attaches to a pair of ribs. Lumbar vertebra are fairly flexible in carnivores, but fairly rigid in herbivores. Not all animals have caudal vertebra, but they are always very flexible.
The rib cage of a quadruped is usually longer vertically (from sternum to spine) than across the body (from a given rib on the left to the corresponding rib on the right). The ribs spring out away from the spine at the top, and then curve back towards the mid-line towards the bottom. The forward sets of ribs connect directly to the sternum, and are called "true ribs". The next set of ribs are connected to the rib in front of them by cartilage, and are called "false ribs". Ribs that do not attach to any other are called "floating ribs". Carnivore ribs tend to be slender, while large herbivore ribs are very wide. The very front end of the sternum is called the manubrium, and marks the "point of the chest". The "point of the chest" is an important landmark, as it remains stationary regardless to the position of the head and neck.
The pelvis is made up of mostly fused bones, and has little flexibility. Looking down the spine, the pelvis is ring shaped, with the top of the pelvis attaching to either side of the sacrum. From this ring, a "wing" called the ilium extends forward on each side. The shape of the ilium varies by species, but it tends to extend outwards more in herbivores than carnivores. The ends of this wing are a bony landmark called the "point of the hip" that is visible on most animals. Near the bottom, on either side of the "ring" are the sockets where the femur attaches to form the hip joint. The portion of the pelvis that extends backwards from the hip joint is called the ischium, which form the buttocks. The ischium is another bony landmark that is visible, or at least discernible, on most animals.
The fore limbs of the quadruped are weight bearing, and so differ from the fore limbs of a biped. The quadruped has an elongated scapula that lies alongside the rib cage, while the scapula of the biped lies behind the rib cage. The clavicle (collar bone) is usually small, or completely absent, and the fore limb is only attached to the body by muscle. The scapula usually slopes forward and meets the humerus at the "point of the shoulder". The "point of the shoulder" is another useful landmark for animal anatomy. The humerus slopes backwards and meets the radius/ulna at the elbow. The radius and ulna are often fused in quadrupeds, which means they cannot rotate their "hands" like humans can. The elbow joint is usually close to the bottom of the rib cage, with the radius/ulna perpendicular to the ground. At the lower end of the radius/ulna is the "knee" joint (the human "wrist"). This joint is usually higher (closer to the body) in herbivores, and lower (closer to the ground) in carnivores.
The knee joint itself contains two horizontal rows of small cube-shaped bones called carpal bones (the human wrist). The pisiform is the most obvious carpal bone, as it projects backwards from the joint. Below the knee are the metacarpals (hand bones), which are often fused in herbivores. Below the metacarpals are the phalanges, (finger bones), which may be split into toes (cow, deer), or remain a single toe (horse). Hoofed animals are literally walking on their fingertips. Carnivores have separate metacarpals, each ending a clawed toe. Carnivores walk on the last bone at the end of the "finger" with large pad supporting the weight from the "knuckle" joint.
The hind limbs of a quadruped provide the power for movement. The femur (thigh bone) attaches to the pelvis with a ball and socket joint. The ball and socket joint allows for movement in multiple directions; inwards (towards the mid-line), outwards, (away from the mid-line), as well as forwards and backwards. The lower end of femur is deeply notched in the front, were the patella (kneecap) fits and slides with the movement of the joint. The tibia meets the femur at the stifle (the human "knee"), which is usually just below the belly-level of a quadruped. The tibia is accompanied along it's length by the fibula, however the fibula is only partial in some animals (ox, horse). At the lower end of the tibia, and fibula if present, is the hock joint (the human "ankle"). Similarly to the knee of the fore limb, the hock is usually higher on herbivores, and lower on carnivores.
The hock joint is made up of small tarsal bones, with the most prominent being the calcaneus (heel bone) which projects outwards and upwards from the back of the joint. The calcaneus bone is very obvious in most quadrupeds, as it will form the point at the back of the hock. Below the hock are the metatarsals (foot bones), which, like the metacarpals of the fore limb, are often fused in herbivores and separate in carnivores. The toes of the hind limb are very similar to the toes of fore limb, except that in carnivores the dew claw is usually absent. The hind feet of a quadruped will also generally be slightly narrower than the front feet, since most quadrupeds carry more of their weight on their fore limbs.
The skull of an animal serves to encase the brain, and houses the eyes, ears, nose, and mouth. The skull is usually only covered by small muscles, so the shape of the head is largely defined by the shape of the skull. The brain in housed in the rounded cranium, near the rear of the skull. Large carnivores will often have a ridge along the top of the cranium, where the chewing muscles attach.
The orbits (eye sockets) are usually on the front of the head for a carnivore, providing better depth perception, and on the sides of the head for a herbivore, providing a wider area of visibility. The zygomatic arch (forming the cheekbones) begins near the orbits and extends out away from the skull, then connects again near the back of the skull. When viewed from the top, the zygomatic arch is often the widest part of the skull.
The mandible (lower jaw) is hinged near the back of the skull, meaning the entire lower jaw is involved when opening and closing the mouth. The upper end of the mandible passes through the space formed by the zygomatic arch. Teeth vary depending on function and age of the animal, but carnivore teeth are usually pointed, while herbivore teeth are comparatively flat. Some herbivores (ruminants) lack front teeth on their upper jaw.
Putting it all Together
Hopefully you now understand a little more about how quadrupeds are put together. So, how do you go about using this new insight? Here are a few suggestions.
The basic structure of mammals, birds, reptiles and amphibians is very similar. Learning about any one of them provides insight into the others. Learning about how animals are put together can provide useful insight into drawing real animals, or creating believable fictional creatures. This article is a starting point to understanding quadruped anatomy, but barely even scratches the surface of information available. If you're interested in learning more, I suggest looking at Animal Anatomy for Artists by Eliot Goldfinger, and An Atlas of Animal Anatomy for Artists by Wilhelm Ellenberger. Both of these books were incredibly useful for putting this article together, and contain more information and drawings than I was able to include here. At the time of this writing, both these books have previews available through Google Books, which include many more illustrations.
Ellenberger, W., Dittrich, H., Baum, H. An Atlas of Animal Anatomy for Artists. Edited by Lewis S. Brown. New York: Dover Publications, 1956. (plates are also available online at http://digicoll.library.wisc.edu/Science/subcollections/VetAnatImgsAbout.html as of 01/25/2011)
Foster, Walter T. Anatomy. Tustin, CA: Walter Foster Art Books, 1970?.
Goldfinger, Eliot. Animal Anatomy for Artists: Elements of Form. New York: Oxford University Press, 2004.
Erica Liszewski is an artist, programmer, and game designer, with way too many pets.
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