Birds

Bird characteristics: Body always covered with feathers; feet (toes and usually tarsometatarsus) covered with scales (thickened skin). Aquatic birds have webbed toes. No teeth; horny beak. Lightweight skeleton in flying birds (many hollow bones), with keel on sternum for attachment of flight muscles (pectoral muscles). No keel in large flightless birds (ostrich, emu, rhea). Completely bony ribs (no rib cartilage). Clavicles fused into single bone, the furculum (wishbone). Numerous neck vertebrae (number varies by species) provide great neck flexibility. Some of the middle thoracic vertebrae fused in some species (chicken); posterior thoracic, all lumbar, and all sacral vertebrae fused into synsacrum, which in turn is fused to the pelvis. Short, flexible tail terminates in stout bone (pygostyle) for support of highly mobile long tail feathers. Wing (arm) skeleton modified for flying (ostrich and penguins evolved from flying ancestors). Wrist joint automatically straightens when elbow joint is straightened; conversely, wrist joint automatically bends when elbow joint is bent. Individual hand and finger bones reduced in number and largely fused together for support of primaries (outer flight feathers). Three digits present; small third digit nonmovable. Short alular feathers attach to movable first digit. Secondaries (inner flight feathers) attach to rear edge of ulna. Three toes point forward and one points backward in most species (e.g., chicken, hawk, crow), or two toes forward and two back (e.g., woodpecker, parrot). Ostrich has two toes per foot. Toes terminate with claws. Male chicken has bony spur covered with horny sheath on tarsometatarsus.

Four-Legged Animals Skeleton & Superficial Muscles (Side View)

Domestic cat characteristics: Rounded skull, short snout. Long whiskers. Large canines, small incisors. Cheek teeth with sharp edges for shearing. Large temporalis and masseter muscles of skull. Eyes shifted slightly forward for binocular vision. Large eyes in domestic cats. Constricted pupil in domestic cats is vertical; round in large cats. Top edge of scapula usually higher than tips of thoracic vertebrae. Small rib cage. Five digits on front limb (thumb reduced); four digits on hind limb—may have very reduced first metatarsal or reduced first digit with claw (dewclaw). Walks on toes. Sharp, curved, retractile claws (which keeps them sharp—they don’t walk on them). Forearm rotates (pronates/supinates). Hairy tail. Very flexible body. Spine (back) straight or arched. Can walk in crouched position, as when stalking prey. Bear characteristics: Large, powerful body; powerful limbs appear relatively short. Rear feet wide. Walks on sole and heel of rear foot and usually on digits of front foot. Five digits per limb with long, curved, nonretractile claws. Front claws longer than rear claws. Large head, small eyes. Small, round, erect, furry ears. Large canines; flat, grinding molars. Short tail. Arched back, high shoulder. Grizzly has most prominent shoulder hump and dished, slightly concave face (in profile). Can have very thick layer of fur. Grizzly and brown bear belong to the same species, but differ in geographical range and size. The giant panda is now considered to be a member of the bear family, not the raccoon family.

Animals with Limb Variations Skeleton & Superficial Muscles (Side View)

Kangaroo characteristics: Forelimb small; has five digits with strong claws. Large, powerful hind limb with long, strong, narrow foot. Muscular thigh; muscle mass of lower leg positioned on upper half, toward knee. In foot, large fourth and smaller fifth digits transmit force during locomotion; first digit missing, small digits two and three bound together by skin. Long tail, thick at base, used for body support at rest and balance during hopping. Fast locomotion is by leaping with hind limbs only. Walking: Hind limbs, forelimbs, and tail in contact with ground at various times. Sitting: Body rests on entire foot and tail; arms hang loosely. Pouch in female opens forward, supported by two long, thin bones. Sea lion characteristics: Webbed flipper-like forefoot and hind foot. Front flipper thicker on front edge. Skin of flipper extends past tips of toe bones, supported by individual cartilages attached to ends of toe bones. Noticeable claws on three middle toes of hind foot; other claws tiny and inconspicuous. Front flipper triangular; hind flipper rectangular. Hind limb can be advanced forward—can walk, but thigh and lower leg encased in skin of abdomen (seals can’t walk—their hind limbs permanently extended backward). Elbow also enclosed in body skin, but forearms are free. Body streamlined (torpedo-shaped) for swimming. Thick insulating blubber layer between skin and muscles. Thick, flexible, muscular neck. Pelvis and femur small. Sternum has forward projection. Small external ears present (absent in seals). Ears and slit-like nostrils can be closed under water. Short, stubby tail. Very short hair; fur much darker when wet. Body uniform in color (no spots). Males larger than females. Closely related to terrestrial carnivores. Pinniped is not scientific classification, but means “fin footed.”

Four-Legged Animals Skeleton & Muscles (Numerous Views)

Horse (equid) characteristics: One digit per foot ending in symmetrical, horny hoof. Walks on very tip of toe. Elongated skull; large lower jaw. Large upper and lower incisors. In side view, neck widens as it approaches shoulder (elongated triangular shape). Mane present, upright on wild species. Tuft of hair often present on forehead (“forelock”). Rear profile of neck straight or arched. Pointed, upright ears. Long, slender limbs. Humerus and femur short; lower portion of limb long, especially forefoot and hind foot (adaptation for speed by shifting weight of muscles mass upward, close to the body). Ulna and fibula reduced and fused to adjacent, weight-supporting bones. Femur has projecting third trochanter on outer side of shaft for attachment of the gluteus superficialis muscle. Barrel-like trunk. Long hair on tail. In same group as tapirs and rhinoceroses.

Basic Body Plan General Overview

There is a basic body plan common to most of the animals presented in this book. At its most obvious, they all have a head, a body, and four limbs. Most are four-legged and stand on all fours, and are described as having front limbs and rear limbs. The front limb is anatomically equivalent to the arm and hand in humans and primates, and the rear limb to the human lower limb. The animals in this book are surprisingly similar in many ways. The head is connected to the rib cage by the neck vertebrae and the rib cage is connected to the pelvis by the lumbar vertebrae. The two front limbs are connected to the rib cage, and the two rear limbs are connected to the pelvis. These units move in relation to one another, establishing the stance, or pose, of an animal. Animals differ primarily in the shape and relative proportions of these structural units, in the position of the wrist, heel, and toe bones when standing and walking, and by the number of their toes. An animal can be visualized as being constructed of a series of simplified, three-dimensional, somewhat geometric volumes (head, forearm, thigh). Each of these volumes has one dimension that is longer than the others. A line projected through the center of the mass of this volume on its longest dimension is called its axis (plural, axes). For the most part, especially in the limbs, these axes follow the skeleton, so that a line drawn through the long dimension of a bone is on, or close to, the axis of the volume of that region (for example, the position of the radius is close to the axis of the forearm). One of the more confusing regions of the body is the volume of the upper arm. The humerus (upper arm bone) is mostly deeply buried in muscle, and lies toward the front of this muscle mass, with the massive triceps muscle located at its rear.

Individual Muscles Attachments, Action, & Structure

The muscles of the head consist of the chewing muscles (temporalis, masseter, and digastric) and the facial muscles (zygomaticus, orbicularis oris, etc.). The chewing muscles are thick and volumetric, and they originate and insert on bone. They open and close the lower jaw, with the action taking place at the jaw joint (temporomandibular joint). The facial muscles are thin. They originate either from the skull or from the surface of other muscles, and they generally insert into other facial muscles or into the skin. When they contract, they move the features of the face (eyes, nose, mouth, ears). As they pull the facial features, they often gather the skin into folds and wrinkles that lie perpendicular to the direction of their muscular fibers (perpendicular to the direction of pull). The mouth region receives the most muscles; therefore, it is the most mobile part of the face. Some facial muscles are so thin that they do not create any direct form on the surface (caninus, malaris, orbicularis oculi), whereas other facial muscles or their tendons may create surface form directly (buccinator, levator labii maxillaris, zygomaticus, and depressor labii mandibularis). Facial muscles are generally more visible on the surface in the horse and the ox than in the dog and feline. The facial muscles, as they move the eyes, nose, mouth, and ears, generate whatever facial expressions animals are capable of producing. . . . • Attachment: A short ligament at the inner corner of the eye, whose inner end attaches to the skull. . . . . . . • Action: Eyelid portion: closes eyelids (blinking), primarily by depressing the upper eyelid. Outer portion: tightens and compresses the skin surrounding the eye, protecting the eyeball. . . . . . . • Structure: The orbicularis oculi is a flat, elliptical muscle consisting of two portions. The eyelid portion lies in the upper and lower eyelids, and the outer portion surrounds the eye and lies on the skull. . . .

Miscellaneous Animals Skeleton

American bison characteristics: Has very long spinous processes on thoracic vertebrae (especially between the shoulders). Four digits with hoofs per limb. Two central toes are large and weight-bearing; vestigial inner and outer toes, with hoofs, are very small and located higher on side of foot and to the rear (they do not articulate with the skeleton and do not touch the ground). Walks on toes. Front half of body develops permanent long hair, especially on the top of the skull, the chin, and the forearms). Rear half looses thick fur cover in the summer, so difference between hair length of front and rear portions of body is very pronounced in warm months, with a clear line of demarcation. Both sexes have horns. African elephant characteristics: Middle of back profile concave/low (convex/high in Indian). Nose extends into long, flexible, muscular trunk with nostrils and one prehensile finger-like projection at tip (two in Indian). Large skull; short nasal bones located high on skull (for attachment of trunk). Brain surrounded by thick, airy bone. Single rounded prominences on top of head (double in Indian). Upper incisors elongated into continuously growing tusks, in both male and female. Very large ears (larger than Indian). Short neck—cervical vertebrae compressed front to back. Rib cage extends to pelvis. Thick, pillar-like columnar limbs (bones in almost vertical straight line) and shoulder and hip sockets face downward, all to support massive body weight. Short feet. All feet have five digits—some inner and outer toes may be reduced and without hoofs. Both front and rear feet have an extra small, elongated bone (prepollex in front and prehallux in rear) just to the inside of the first digit. Front foot has four or five hoofs (five in Indian). Hind foot usually has three, four, or five hoofs (four or five in Indian). Thick elastic pad on sole of foot. Foot in life somewhat cylindrical or conical. When the animal is lying on belly, knee touches ground (femur directed downward); lower leg continues straight back. Long tail has wispy tuft of coarse hair.