22 



t-RACTURfiS. 



(1.) The hom core projecting from the frontal hone does not 

 develop until after hirth. Ahout 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. 



(2.) The horn-secreting membrane is formed by the skin, which under- 

 goes special development around the base of the horn and comes to 



Fig. 9. — (.', horn; P, modified skin forming the keratogenous membrane; 

 (), horn core, exhibiting a double sinus. 



resemble that of the coronary band, from which the hoof or claw is 

 secreted. The liand is about one-fifth of an inch in breadth. The 

 papillffi 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 distri- 

 buted to the enlarged papillae. The great vascularity of these parts 



