Horns and antlers are found in the order Artiodactyla (cattle, sheep, deer, giraffes, and their relatives). Several other types of mammals have similar head structures, but true horns, originating from the frontal bone of the skull and found only among the Bovidae (cattle, antelopes, buffalo), consist of a bony core enclosed by a tough keratinized epidermal covering or sheath. True horns are not branched, although they may be curved. Horns grow throughout the life of the animal and are used for defense, display, and intraspecies combat (e.g., contests between males for mates). A variation of the true horn is the pronghorn, found on the North American pronghorn (Antilocapra americana) in the artiodactyl family Antilocapri-dae. A pronghorn branches and its epidermal sheath is shed on an annual basis; the sheath on a true horn is not shed. Antlers are found among the Cervidae (deer, caribou, moose, and their relatives). Mature antlers are entirely made of bone and are branched. They develop from buds covered by in tegument that is richly innervated and vascularized, called velvet. As the antlers grow the velvet dies and the animal usually rubs it off on tree trunks. Antlers are used for combat between males for mates. After the breeding season they are shed and replaced by a larger pair the next year; this continues until they reach their full growth. The small bony horns of giraffes (Giraffe camelopardalis) originate from the anterior portion of the parietal bones. Because they do not arise from the frontal bones they are not considered true horns. Giraffe horns are covered by furred skin and persist throughout life. Another type of horn is found on the rhinoceroses of the order Perissodactyla, the only living mammals outside the artiodactyls with a horn. The rhinoceros horn is centered over elongated nasal bones, but it lacks a bony core. It is a solid mass composed of dermal cells interspersed with tough epidermal cells.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.