Skeletal Structure
 Rodent skull, ventral view.
The two small dark holes near the middle of the skull are called foramens, through which passes nerve bundles and
blood vessels. The large opening at the back of the skull is called the
foramen magnum, and this serves as a conduit for the spinal cord. To
either side and forward lie the occipital condyles.
 Rodent skull, dorsal view.
The part of the skull that contains the brain is called the braincase.
The two large holes seen are called orbits, or eye sockets. The orbits
in flying squirrels are more side-facing than forward-facing. As a
general rule, prey animals have side-facing orbits and predators have
forward-facing orbits.
The outer rings surrounding the orbits are called zygomatic arches, and they form the cheekbones and extend to the back along each side of the skull, attaching above and slightly forward of the ears. The bones of the muzzle are called the rostrum. The ridge running lengthwise along the top of the braincase is called the sagittal crest.  Rodent skull, lateral view.
As is the case with all rodents, a large gap, or diastema, exists between the incisors and "cheek
teeth".
The forward portion of a flying squirrel's jaw is employed to tear and cut food, while the rear portion is used to crush and grind food. There are two major muscle groups in the rodent skull - the masseter and the temporalis muscle groups. The temporalis muscles attach to the rear portions of the lower jaw, run beneth the zygomatic arches, and run up to and attach to the sides of the cranium. A lateral attachment anchors temporalis muscles along the sides of the cranium near the temples. A dorsal attachment anchors temporalis muscles along the sagittal crest near the top of the cranium, which provides a great deal of power to the forward portion of the jaws (incisors). The masseter muscles attach between the zygomatic arches and the rear portions of the lower jaw. These muscles greatly enhance side-to-side grinding action and overall crushing strength (pre-molars and molars).  Southern flying squirrel skull (expanded) at left; northern flying squirrel skull (expanded) at right.  Southern flying squirrel skull (expanded) at top; northern flying squirrel skull (expanded) at bottom. As illustrated above, the skull is comprised of two parts - the cranium and the jaws. All jawed vertebrates, with the exception of birds, have teeth. Note the large gap, or diastema, between the incisor and cheek teeth.  Illustrated above are partial skeletons of the southern flying squirrel (top) and northern flying squirrel (below).  Right hind leg bones of a northern flying squirrel.  The x-rays of a southern flying squirrel shown here illustrate the fine bone structure required for a gliding mammal.  Pictured above is a much-enlarged photo of a northern flying squirrel's baculum. The baculum made of bone, and is employed as a an aid in mating - it might be important in stimulating the female and to induce the "hormone surge" required for ovulation. It is also felt that it aids in "strengthening" the penis. Present in most mammals, and all rodents, it is not found in lagomorphs, primates and marsupials. |