Anatomy of the horns. The horns form organs of defence, and project on either side of the frontal bone at the poll. Each consists firstly of a bony basis generally known as the horn core; secondly, of a horn-secreting membrane; thirdly, of a horny sheath, the horn properly so called.
(1.) The horn core projecting from the frontal bone does not develop until after birth. About the third month a little prominence appears under the skin, which, as it develops, assumes a conical shape, and may be seen to be covered with a horny substance. In proportion as the horn core grows, there develops within it a cavity which may either be of a simple character or divided by a longitudinal partition. This communicates with the frontal sinus, a fact which explains the collection of pus in the sinuses as a result of injuries to the horns. The sinus of the horn core does not exist in young animals, and is not completely developed before the third or fourth year of life.
Fig. 9.—C, horn; P, modified skin forming the keratogenous membrane; O, horn core, exhibiting a double sinus.
(2.) The horn-secreting membrane is formed by the skin, which undergoes special development around the base of the horn and comes to resemble that of the coronary band, from which the hoof or claw is secreted. The band is about one-fifth of an inch in breadth. The papillæ of the dermis are specially developed at this point, and the epithelium which they secrete eventually forms the horn.
The internal surface of the growing horn is adherent to the horn core through the medium of another tissue formed by a specially differentiated periosteum which is continuous with the periosteum covering the frontal bone. It is not a true periosteum, but a vascular tissue formed of papillary layers analogous to those of the podophyllous tissue of the ox’s claw or horse’s hoof.
This keratogenous membrane receives a rich vascular supply from the arterial circle formed at the base of the horn core by a division of the external carotid, the blood conveyed by which is freely distributed to the enlarged papillæ. The great vascularity of these parts explains why lesions of the horns are often followed by such profuse bleeding.
(3.) The horn secreted by the papillæ of the horn band (which is analogous to that of the coronary band of the horse) forms a cone varying in its curve in various breeds. Its base is hollow, and contains little depressions holding the papillæ from which the horn is secreted. From its base up to the end of the horn core the walls progressively increase in thickness. From this point it is solid; in a fully-grown horn the bone does not extend more than one-half or two-thirds of the entire length.
In the adult, the development of the horns varies with different breeds and is affected by sex. In the bull the horns are short, but in the cow and ox long. Short and fine in animals of improved breed like the Durham, they are long and thick in breeds of working oxen.
Injuries affecting the horns are of three classes, determined by the part affected.