BIOLOGY Volume 2 - A Guide to General Biology - 2004

18. THE LOCOMOTOR SYSTEM IN ANIMALS

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

The basic Regions of the Skeleton are similar in all mammals. They can be divided into two main parts.

1. The Axial Skeleton, consisting of the Skull, Vertebral Column, Ribs, and Sternum.

2. The Appendicular Skeleton, which includes the pectoral (shoulder) and pelvic girdles, as well as the forelimbs and hindlimbs.

18.3.1. Axial Skeleton

Skull

The skull consists of the cranium—to which the fixed upper jaw is fused—and the lower jaw, which is articulated via joints. Accordingly, Muscles connect the lower jaw to other BONES OF THE skull. The cranium is formed by several flat bones firmly joined together by immovable sutures. It protects not only the Brain housed within, but also the olfactory Organs, the middle and inner ears, and the eyes. On the posterior surface of the skull, There are two rounded, smooth projections called the occipital condyles, which form a joint with the first cervical vertebra (the atlas), enabling the animal to raise and lower its HEAD.

Vertebral Column

The vertebral column (spine) is the main axis of the body. It consists of a chain of vertebrae separated by cartilaginous intervertebral discs (Fig. 18.4). Ligaments connect the vertebrae to one another, preventing displacement while still allowing a degree of movement, so that the spine as a whole possesses considerable flexibility. THE VERTEBRAL COLUMN also serves to protect the Spinal Cord. The vertebrae bear numerous processes for Muscle attachment, and the contraction of these muscles allows the spine to bend forward, backward, or sideways.

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Fig. 18.4. Rabbit skeleton (left lateral view). The vertebral centra are subjected to compression, whereas the Ligaments and Muscles connecting the vertebrae are subjected to tension. The abdominal wall musculature prevents the limb girdles from spreading apart under METABOLISM/18.html">The Influence of gravity.

The total number of vertebrae varies among different mammals. Nevertheless, five distinct regions are recognized in the vertebral column of all mammals (Table 18.1).

Table 18.1. Types of vertebrae and their numbers in various mammals

The vertebrae across all regions share a common structural plan. The Structure of a typical vertebra is shown in Fig. 18.5. Two facets (articular surfaces) on the anterior side of the vertebra (prezygapophyses) articulate with facets on the posterior side (postzygapophyses) of the adjacent vertebra in front of it. This arrangement allows the vertebrae to articulate with one another without creating a completely rigid structure, as the main articular surfaces permit slight movement between adjacent bones. A small notch is located beneath each anterior and posterior facet. The posterior notch of one vertebra lies directly opposite the anterior notch of the vertebra immediately behind it, thus forming the intervertebral foramina through which Spinal Nerves pass.

Fig. 18.5. Typical mammalian vertebra (anterior view).

All vertebrae are also characterized by the presence of a neural spine (spinous process) and transverse processes, to which muscles attach. The solid central portion of the vertebra is the centrum (vertebral body), above which lies the neural arch, enclosing the spinal cord.

Despite significant similarities among all vertebrae, their structure varies across different regions of the spine. This is because body weight is distributed unevenly along the length of the column, and the vertebrae become modified to suit the specific Functions performed by each region.

When a rabbit is standing, its spine is supported by the forelimbs and hindlimbs, with body weight distributed between them. Under these conditions, compressive forces act upon the vertebral centra, while tensile forces act upon the dorsal ligaments and muscles located above them (Fig. 18.4).