THE ENDOSKELETON OF A VERTEBRATE ANIMAL: THE RABBIT

The endoskeleton of a vertebrate consists of :—

The skull and vertebral column.

The ribs and the breast bone.

The limbs.

The bones known as the limb girdles to which limbs are jointed.

The Vertebral Column of the Rabbit

The vertebral column of the rabbit consists of about forty-five small bones or vertebne which together form a hollow tube in which the spinal cord runs. The vertebras are bound to each other by fibrous tissue so that although the column as a whole is fairly flexible the vertebra; themselves can only move slightly on each other. They are separated by pads of cartilage known as intervertebral discs.

The Structure of a Typical Vertebra

One of the anterior thoracic vertebra may be taken to show the structure of a typical vertebra. The vertebra is pierced by a large hole, the vertebral foramen, through which the spinal cord passes, and the floor of the foramen is formed by a thick bony mass called the centrum. An arch of bone, the neural arch, forms the sides and roof of the vertebral foramen. From the top of the neural arch the neural spine projects backwards and upwards. A small projection, the transverse process, juts out on each side of the neural arch and forms a basis of attachment for muscles. The neural arch is notched in front and behind for the passage of the spinal nerves outwards from the spinal cord. Above ‘ these intervertebral notches at the front and back of the neural arch are two small projections, termed the anterior and posterior zygapophyses respectively. The zygapophyses of one vertebral articulate with those of the vertebra in front and behind, and limit the rotation and backward movement of the vertebra. The articulating surfaces of the anterior zygapophyses face upwards and are slightly turned in to one another, while the surfaces of the posterior zygapophyses face downwards and outwards.

The Cervical Vertebrae

In the rabbit and all other mammals there are seven cervical vertebra;. These have short neural spines, and are characterized by the presence of a hole in each transverse process, called the vertebrarterial canal, through which the vertebral artery passes. Beyond the canal the outer part of the transverse process divides into two short parts. The first two cervical vertebrae differ from the others in being specially modified so as to allow the free movement of the skull on the vertebral column. The first cervical vertebra, called the atlas , A, is a ring of bone with a very large neural foramen. The transverse processes of the atlas are broad and flat. At the front of the atlas there are two large hollows which articulate with a pair of knobs or condyles at the base of the skull. These joints only permit an up and down or nodding movement of the skull. The neural spine of the atlas is a slight ridge, and at the back of this vertebra are small facets which articulate with similar facets on the second cervical vertebra. The centrum of the second vertebra, which is termed the axis , B, is broad and produced in front into a spine-like projection, the odontoid process. The central cavity of the atlas is divided into upper and lower halves by a ligament, and the odontoid process projects into the lower half. Consequently the atlas can twist about the odontoid process, which acts as a pivot. The odontoid process represents part of the centrum of the atlas which has become detached during development, and has fused with the centrum of the axis. The axis bears a large neural spine and small transverse processes which are perforated by vertebrarterial canals. Posterior zygapophyses occur on the hinder edge of the neural arch of the axis.

Lumbar Vertebrae

There are six or seven lumbar vertebra; which are large, with thick centra, and have well-developed transverse processes projecting forwards and downwards from the centrum. A short projection, the hypapophysis, occurs on the under side of the centra of the first and second lumbar vertebra;. The metapophyses are paired processes which project above the anterior zygapophyses, while the anapophyses are a pair of small processes beneath the posterior zygapophyses. These extra projections provide attachment for muscles, for these vertebras have to support the heavy abdominal region of the body. A neural spine projects from the dorsal side of the neural arch of each lumbar vertebra.

Sacral Vertebrae

The sacral region consists of three or four vertebras, the first two of which are completely fused together, whilst the others are less completely fused with them. The whole fused mass is termed the sacrum. The transverse processes of the first sacral vertebras are large and wing-like, and are firmly attached to the pelvic girdle, but the other sacral vertebras do not join it. Paired holes on the under side of the sacrum show where nerves leave the spinal cord. The neural canal is small in the first sacral vertebras and becomes narrower behind, until it disappears in the caudal region.

Caudal Region

There are about sixteen caudal vertebras, which decrease in size from the sacrum backwards, gradually losing the transverse processes, neural spines and zygapophyses. Finally the neural arch is lost and the terminal vertebras consist only of small rod-like centra.

The Skull

The skull consists of two parts, a posterior cranial region and an anterior facial region. The cranial region is formed by a box of bone, termed the cranium, which bears on each side a smaller box containing the ear, and termed the auditory capsule. The cranium contains the brain, and at its hinder end is a hole, the foramen magnum, through which the spinal cord passes. At the side of the foramen magnum are a pair of curved elevations, the occipital condyles, which fit into two grooves on the anterior surface of the first vertebra. The facial region of the skull is formed by the bones which support the nose and by the upper jaws. At the anterior end of the frontal region is a hole forming the opening to the nasal passages which lie above the roof of the mouth. The roof of the nasal passages is formed by a pair of long narrow bones, termed the nasal bones. Lying in front of the cranium are a pair of deep hollows, which form the orbits or eye sockets. On each side of the skull a flat bar of bone, termed the zygomatic arch, joins the facial 2nd cranial regions and forms the lower edge of the eye sockets. The lower jaw or mandible consists of a pair of flat bones, which are joined in front, and at their hinder ends are articulated to sides of the cranium just behind the orbit. The lines of contact where the bones of the skull meet are called sutures ; the bones have wavy or zigzag edges which interlock with each other. In most mammals the bones grow together so that the sutures cannot be seen, but in the skull of the rabbit the bones remain separate. Embedded in the base of the tongue are several small bones which form the hyoid apparatus.

Ribs

There are twelve or thirteen ribs which lie in the wall of the thorax and are movably attached to the thoracic vertebras at the dorsal ends. Ventrally they curve round to meet a series of bones called the sternum lying in the mid-ventral line. The ribs and sternum form a bony cage which protects the lungs and heart, and which takes part in the breathing movement of the animal.

The upper end or capitulum of each rib fits into the articular surface formed between the centra of the thoracic vertebras. A short way along the rib from the capitulum is another articu lating surface, the tuberculum, which is attached to the transverse process of the corresponding vertebras. The last three ribs are without tubercula.

At the lower end of a rib is a bar of cartilage, which in the case of the first seven ribs is attached to the sternum. The remaining ribs are not attached to the sternum; the eighth and ninth join the ribs in front, and the last three are free at the ventral end. The sternum of the rabbit consists of seven short rod-like bones called the sternebras the last of which bears a plate of cartilage.

The Vertebrate Limb Plan

In all vertebrates except fishes the limbs are constructed on a similar plan known as the pentadactyl limb because it ends in five digits. The upper part of the limb is supported by a single bone known as the humerus in the fore leg and the femur in the hind leg. This bone is jointed distally to two bones lying parallel to each other, and termed the radius and ulna in the fore leg and the tibia and fibula in the hind leg. Beyond these paired bones are nine small bones forming the carpals or wrist bones of the fore leg and the tarsals or ankle bones of the hind leg. Articulating with the small bones of the wrist or ankle are five rod-like bones, the metacarpals or metatarsals, which lie in the palm of the hand and the sole of the foot respectively, and are jointed distally with five fingers or toes. The small rod-like bones of the fingers and toes are known as phalanges ; there are two phalanges in the thumb and big toe, and the remaining digits have three phalanges each. Such is the primitive arrangement of the bones in the pentadactyl limb, which in many animals has become altered by the fusion or disappearance of certain bones.

Each limb is jointed at its upper end to a supporting framework of bone within the body, known as a limb girdle, and which in the lower vertebrates has the shape of an inverted arch. The limb girdle of the fore limb is named the pectoral girdle, and that of the hind limb the pelvic girdle. Each girdle is made up of two halves joined in the mid-ventral line, and to each half there is an upper and lower region. The upper region is formed by a single bone, termed the scapula, in the pectoral girdle and the ilium in the pelvic girdle, while the lower region consists of two parallel bones lying one in front of the other. The anterior of these is the clavicle in the pectoral girdle and the pubis in the pelvic girdle ; the posterior is the coracoid in the pectoral girdle and the ischium in the pelvic girdle. The limb girdles are to be found in their primitive state only in the lower vertebrates, and show various modifications of this state in the higher vertebrates.

The Pectoral Girdle of the Rabbit

In the rabbit, as in most mammals, the pectoral girdle is incomplete, and is formed chiefly by the upper or scapular part of the inverted arch. The scapula or shoulder blade is a triangular plate of bone, the apex of which is directed forwards and downwards. At the lower end is a hollow, the glenoid cavity, into which the head of the humerus fits. Near to the glenoid cavity is a small hook-shaped projection of bone which represents the coracoid. A ridge termed the spine runs along the outer surface of the scapula and ends in two pointed processes near the lower edge of this bone. The scapula is bound by muscles and ligaments to the outer side of the ribs, and the whole thus forms an elastic yet firm support for the fore limb. The clavicle is represented by a small rod of bone attached to a ligament running from the sternum to the lower end of the scapula.

The Fore Limb

The humerus is a long bone which articulates by means of a rounded head with the glenoid cavity of the pectoral girdle. In front of the head the bone is grooved for the attachment of the biceps muscle. At its distal end the humerus ends in a grooved, pulley-like surface to which the radius and ulna are jointed. The radius lies in front of the ulna at the elbow, but lower down it crosses the ulna so that at its distal end it lies on the inner side of this bone. The ulna is longer than the radius, for it is prolonged beyond the joint surface at the elbow into a curved projection, the olecranon process. When the arm is straightened the olecranon process fits into a fossa in the humerus, and so prevents the further backward movement of the forearm. The wrist consists of nine small carpal bones which articulate distally with five rod-shaped bones, the metacarpals. These are jointed to the five digits. The thumb, which lies on the inner side of the limb, is shorter than the other digits, and consists of only two small bones or phalanges, while the remaining digits have three phalanges each. Each digit ends in a sharp claw.

The Pelvic Girdle

The pelvic girdle is firmly attached to the transverse processes of sacral vertebrae, and consists of two halves which are joined beneath the vertebral column. Each half consists of three bones, the ilium, ischium and pubis, which are only recognizable as separate bones in the young rabbit and later fuse together. Each half of the girdle has on the outer side a deep hollow, the acetabulum, for articulation with the head of the femur. The dorsal part of each half of the girdle is formed by the ilium which is attached to the sacrum. The ischium and pubis form the lower part of each half of the girdle, and enclose a large hole between them, termed the obturator foramen, through which nerves and blood vessels pass. The pubis joins its fellow of the other side so as to unite the two halves of the pelvic girdle.

The Hind Limb

At its proximal end the femur bears a rounded projecting head which fits into the acetabulum so as to form the ball-and-socket hip joint. Near the head of the femur are three rough projections or trochanters which serve as attachments for certain leg muscles. At its distal end the femur bears two rounded knobs or condyles which articulate with the tibia and are separated by an intercondylar groove. The tibia is the largest bone of the shin and bears at its proximal end two oval surfaces for articulation with the condyles of the femur. The fibula is a small bone which lies on the outer side of the tibia and becomes fused with it distally. At the front of the knee joint is a small, round, flat bone, the knee cap or patella, which represents part of a tendon that has become converted into bone, and is used for extending the lower half of the limb. The small bones of the ankle are termed tarsals, and only six are present. One of these on the inner side of the ankle is prolonged backwards into a process that forms the heel. The foot of the rabbit has four toes only, for the digit corresponding to the big toe has disappeared. Each of the four digits consists of three phalanges. The last phalanx of each digit bears a claw.

Pronation and Supination

In the fore limb of the rabbit the thumb is on the inside of the limb and the palm of the hand is turned downwards. The limb is said to be in the prone position, and is fixed so that the hand cannot be twisted round. The radius lies in front of the ulna at the elbow, and on its inner side at the wrist. In man the arrangement of these bones is somewhat similar ; the radius lies on the outside of the ulna at the elbow, crosses over the ulna and lies on its inner side at the wrist. But man differs from the rabbit in being able to twist the hand from the prone position, so that the palm faces upwards and the radius and ulna alter their position so that they lie parallel instead of crossing. The hand is now said to be in a supine position, and the uncrossing movement of the radius is termed supination. The movement from the supine position into the prone position is called pronation.

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