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SKELETAL SYSTEM
The skeletons of different species of vertebrates share many similarities. Since the rat skeleton is small
you may find it easier to understand the basic organization if you compare the articulated skeleton of the
rat to that of a cat. It may also be helpful to examine individual cat bones and note the many similarities
and differences between the two species.
The skeleton of vertebrates is composed primarily of bone. Cartilage covers articular surfaces between
bones and connects the ribs to the sternum. The skeleton is divided into two major parts:
• The axial skeleton includes the skull, mandible, hyoid, ribs, sternum, and vertebrae
• The appendicular skeleton includes the girdle, limb and feet bones
Skull. The skull which is part of the axial skeleton has two major regions (Fig 2.1). The facial region
houses the nose, eyes, and provides support for the jaw. It is elongate in mammals that depend on their
sense of smell. The more posterior cranial region of the skull houses the brain and ear. The size of this
region depends on brain volume. At the back of the skull is the foramen magnum through which the
spinal cord passes into the vertebral column. On either side of the foramen magnum are the occipital
condyles, which articulate with the first cervical vertebra. Examine the rat skull and trace the lines or
sutures between the bones. Sutures make it possible to identify individual bones. Identify the following:
Premaxillary and maxillary. These
bones are associated with the teeth.
The large incisors are located in the
premaxillary bones and the molars are
in the maxillary bones.
Nasal. These bones form the dorsal
wall of the nasal cavity.
Frontal. These bones form the roof of
the anterior portion of the skull and the
lateral wall of the eye socket.
Zygomatic arch. The arch encloses
the ventrolateral side of the orbit. It is composed of three bones: the maxillary at the anterior end, the
zygomatic bone in the middle and the temporal bone forming the posterior margin. An expanded
tympanic bulla with an opening to the inner ear lies at the base of the temporal bone.
Parietal & Interparietal. These bones and the temporals form the dorsal and lateral wall of the cranium.
Occipital. This bone has the occipital condyles and foramen magnum on its caudal surface.
Palatine. These bones form the roof of the mouth and the ventral surface of the facial portion of the skull.
Sphenoid. These bones form the ventral wall of the cranial cavity, that houses the brain.
Mandible. Two dentary bones fuse anteriorly to form the lower jaw or mandible which provides
support for two types of teeth: incisors for gnawing and molars for grinding.
The many openings in the skull bones provide for the passage of nerves and blood vessels. Flat spaces
and ridges provide surfaces for attaching muscles that support the skull and jaw.
Figure 2.1. Left, lateral vie w of a rat skull.
