The skull is the skeleton of the head, and determines the overall shape of the head in life. It consists of the cranium or braincase in the back, the face in front, and the lower jaw. Major structures overlying the skull and influencing its form are the nostrils, the lips, and the chewing muscles (two on each side—four total). The cranium, which houses the brain, is a rounded form. It can be covered to a greater or lesser degree by the encroachment of the chewing muscles of the temple (the temporalis muscles) toward the midline on the top of the head. The muscles are farthest apart in animals with horns and antlers and in most primates (with the cranium exposed), and they touch on the midline in horses, domestic cats, and some breeds of dog. When the muscles are very developed, a raised bony ridge forms on the midline of the cranium, toward the back, which provides additional surface area for attachment of the temporalis muscle. This feature is typically seen in some dog breeds, large cats, and the male gorilla.
The back (or base) of the skull has a transverse bony ridge, the occipital ridge, with a protuberance on its midline called the occipital protuberance. The ridge can occasionally become prominent and define the separation between the back of the head and the neck, especially in carnivores. The protuberance may also be prominent, forming an important bony landmark. Other subcutaneous areas of the skull that directly create surface form are the bones of the forehead, the top and back of the cranium (when not covered by muscle), the zygomatic arches (cheekbones), the bones of the orbit, which surround the eye, and the top of the ridge of the snout, descending to the nose. The zygomatic arch, often the widest part of the head, is continuous with the orbit. In carnivores, the zygomatic arch is large and projects outward considerably. The back edge of the orbit in carnivores is missing in the skeleton, but it is formed by a ligament in life.
The mandible, or lower jaw, hinges at the back end of the skull, just in front of the ear hole. Therefore, when the mouth is opened, the entire bottom half of the head is involved, not just the mouth area. The convergence of the two sides of the lower jaw toward the front (the chin) is the key to understanding the shape of the bottom of the head. Also, note whether the lower edge of the lower jaw is straight or curved On side view).
The teeth vary in number, size, and their absence or presence according to the different animal groups. They are, when all are present (from front to back), the incisors, then sometimes a space, the canines, then sometimes a space, the premolars, and finally the molars. Some of the more dramatic variations on this pattern are the absence of the upper incisors and all the canines in most of the hoofed artiodactyls (bovids and deer), and the extreme development of the upper incisors in the elephant (the tusks), the upper canines in the walrus, the self sharpening incisors in the rodents, and the canines in the warthog.
Horns and antlers are sometimes found in both males and females of the same species, and sometimes only in the males. Horns are present in the bovid group of artiodactyls, and consist of a bony core (an extension of the skull) covered by a horny sheath, similar in material and origin to fingernails. They are permanent and grow throughout life. Antlers are present in the cervid, or deer, group of artiodactyls. They are bony structures but are shed every year and new ones are regrown. While they are growing, they are covered with a soft skin called velvet. The velvet dies when the antlers are fully grown for that year and is then scraped off by the animal. The pronghorn antelope is notable in that it has a true horn with a bony core and sheath, but the sheath is shed every year (a new sheath grows under the old one before it is shed). Horns and antlers typically arise from the skull either directly above the eye, or more commonly from above and behind the eye. The giraffe has a unique arrangement of its three horns, with a horn above and behind each eye and a single one on the midline in front of them. These horns consist of bone permanently covered with skin and fur. The rhinoceros, related to the horse group and not the artiodactyls, has one or two horns, made up of densely compressed hair with no bony core.
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