BIOLOGY Volume 2 - A Guide to General Biology - 2004

18. THE ANIMAL SKELETOMUSCULAR SYSTEM

18.3. Anatomical Structure of the Mammalian Skeleton (Using the Rabbit as an Example)

18.3.2. Structure and Function of Rabbit Vertebrae

Cervical vertebrae

There are 7 cervical vertebrae, with the 3rd through 7th sharing an identical Structure (Fig. 18.6, A). Their vertebral bodies are relatively small, designed to withstand compressive forces, and their neural spines are short to serve as attachment sites for Neck Muscles. Some of these muscles extend from the cervical vertebrae to the thoracic vertebrae, holding the neck in an elevated position, while others run to the Base of the Skull to support the HEAD. On either side of the vertebral body lies a foramen formed by the fusion of the cervical rib with the transverse process. Together, these foramina form the vertebral canal. As the name implies, this canal transmits the vertebral artery, which supplies Blood to the Brain. This arrangement effectively protects a vital blood vessel located in the vulnerable region of the neck.

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Fig. 18.6. A. Fifth cervical vertebra of a rabbit (anterior view), showing the characteristic foramina for the vertebral Arteries. B. Atlas of a rabbit (anterior view), noting the absence of a vertebral body and the presence of anterior articular facets. C. Axis of a rabbit (lateral view), distinguishing features include the dens (odontoid process) and the forward-directed neural spine.

The first two cervical vertebrae differ markedly in structure from the rest, as they support the head and facilitate its movement in multiple planes. The first cervical vertebra, the atlas (Fig. 18.6, B), lacks a body and prezygapophyses, and its neural spine is reduced to a barely noticeable ridge. On the anterior surface of the atlas are two depressions—the anterior articular facets—which articulate with the convex occipital condyles to form a hinge joint. Thus, the vertebra Supports The Skull and allows the animal to raise and lower its head. Wide, flattened transverse processes provide a large surface area for the attachment of the muscles involved in these movements.

The second cervical vertebra is the axis (Fig. 18.6, V); it features a forward-pointing, finger-like projection called the dens (or odontoid process), formed by the fusion of the bodies of the atlas and axis. The dens projects into the ring of the atlas, where a transverse ligament separates it from the vertebral canal (Fig. 18.6, B). This creates a pivot joint that allows the head to rotate from side to side. Muscles responsible for these movements are located on the sides of the neck; they extend forward from the neural spine of the axis to attach to the transverse processes of the atlas (Fig. 18.7). The axis has no prezygapophyses.

Fig. 18.7. Arrangement of cervical muscles between the atlas and axis in the rabbit.

Thoracic vertebrae

These vertebrae feature long, posteriorly directed neural spines and short transverse processes. The latter bear rounded costal facets (demifacets), while the lateral surfaces of the vertebral body feature anterior and posterior demifacets. Both types of facets serve for rib attachment (Fig. 18.8, A). The ends of the Ribs that articulate with the thoracic vertebrae possess two prominences—the head (capitulum) and the tubercle. The tubercle articulates with the costal facet of the transverse process, while the head articulates with two demifacets on the vertebral body. The joint here is quite complex: the anterior demifacet of one vertebra, together with the posterior demifacet of the vertebra immediately in front of it, forms a complete articular socket. The head of the rib fits into this depression, articulating simultaneously with two vertebrae. The thoracic vertebrae support the ribs, but due to The complexity of these articulations, their mobility is limited. They allow for some slight movement forward and sideways, but overall, the thoracic vertebrae are the least mobile in the spinal Column.

Fig. 18.8. A. Thoracic vertebra of a rabbit, showing the long neural spine and demifacets. B. Lumbar vertebra of a rabbit (left view). Hypapophyses are not shown; where present (on the First and Second lumbar vertebrae), they appear as small projections on the ventral surface of the vertebral body. C. Sacrum of a rabbit (dorsal view).

Lumbar vertebrae

The vertebrae of this region experience the greatest mechanical loads due to gravity and locomotion. They must not only provide structural support for the body but also allow for bending backward, forward, and sideways, as well as twisting the torso. It is hardly surprising, therefore, that the largest back muscles attach in this region and that the vertebrae here have undergone The most significant adaptive modifications. The bodies of these vertebrae are relatively short, but they, along with their neural arches, are exceptionally massive. The specific STRUCTURE OF THE lumbar vertebrae enhances spinal flexibility. The transverse processes are long and broad, directed forward and downward. These vertebrae also feature additional processes for Muscle attachment—metapophyses, anapophyses, and hypapophyses (Fig. 18.8, B)—which also articulate with one another, ensuring the correct relative positioning of the vertebrae under heavy loads in this region of the spine.

Sacral vertebrae (sacrum)

Four sacral vertebrae fuse to form a broad bone known as the sacrum (Fig. 18.8, C). The first sacral vertebra possesses well-developed transverse processes that are rigidly fused with the Pelvic Girdle. When the animal is standing, it is the sacrum that transfers body weight to the pelvic girdle and limbs. During locomotion, the propulsive force generated by the hind limbs is transmitted through the pelvic girdle and sacrum to the other sections of the Axial Skeleton.

Caudal vertebrae

The number of caudal vertebrae varies widely among mammal species (Table 18.1), depending on tail length. Typically, toward the tip of the tail, the transverse processes, neural arches, and articular processes (zygapophyses) diminish in size and gradually disappear, leaving the terminal caudal vertebrae represented solely by tiny vertebral bodies. In humans, this region consists of four fused vertebrae forming a single bone called the coccyx, which is externally imperceptible.

RIBS AND Sternum

A rib is a flattened, curved bone. Its bifurcated dorsal end features a head and a tubercle, both of which articulate with the thoracic vertebrae. The resulting joints allow the ribs to move during Respiration. Together with the sternum and the thoracic spine, the ribs form the rib cage (Thorax), which protects the Lungs, Heart, and major Blood Vessels (Fig. 18.9).

Fig. 18.9. Rabbit thoracic vertebra articulated with two ribs (anterior view).

In the rabbit, the ventral ends of the first seven pairs of ribs attach to the sternum via costal cartilages. The sternum is a flat bone resembling the shape of an elongated kite. These are known as true ribs. The ventral ends of the next two pairs of ribs are also attached to the Cartilage of the seventh pair. The ventral ends of the last three or four pairs remain free—these are the so-called floating ribs (Fig. 18.4).