586 



INTEGUMENTARY SYSTEM 



bones of the skull. In most instances the frontal bone alone is involved. Each 

 bony outgrowth pushes the epidermis before it. The epidermis then responds 

 by producing a highly keratinized, horny substance around the outgrowing 

 bone. The result is the formation around the bony outgrowth of an unbranched 

 cone (or horn) of cornified epidermal material (fig. 275A). This type of 

 horn grows continuously until the mature size is reached. If removed, this 

 type of horn will not regenerate. Horns of this structure are found in sheep, 

 goats, cattle, and antelopes. 



The horns of the pronghorn, Antilocapra americana, are somewhat similar 

 to those of cattle, with the exception that the external, keratinized, slightly 

 branched, horny covering, overlying the bony core, is shed yearly, to be 

 replaced by a new horny covering (fig. 275B). 



On the other hand, the antlers of the deer offer a different developmental 

 procedure. A new bony core is formed each spring which grows and forms 

 the mature antler. As this hard, bony antler matures during late summer 

 and early autumn, the outside covering of epidermis (i.e., the velvet) even- 

 tually atrophies and drops off, leaving the very hard, branched, bony core or 

 antler as a formidable fighting weapon for use during the breeding season 

 (fig. 275C). When the latter period is past, the level of the male sex hormone 

 falls in the blood stream, which brings about a deterioration of the bony 

 tissue of the antler near the skull. This area of deterioration continues until 

 the connection to the frontal bone becomes most tenuous, and the antlers fall 

 off, i.e., are shed. (See Chap. 1, p. 27.) 



The horns of the giraffe are simple, unbranched affairs which retain the 

 velvet or epidermal covering around a bony core. The horns of the rhinoceros 

 are formidable, cone-shaped, median structures (one or two), composed of 

 a keratinized, hair-like substance. These horns are located on the nasal and 

 frontal bones. (For a discussion of horns in the Mammalia, see Anthony, 

 '28, '29.) 



OF W \ 



ment ^^,^ 1 



ANTlErIJ \; EPIDERMIS 

 ■HED ' ' ' 



Fig. 275. Horns of mammals. (A) Cow. (B) Prong-horn antelope. 

 (C) White-tailed deer. 



