36 THE HORSE IN HEALTH AND DISEASE 



and are covered in the living animal with mucous membrane. 

 The anterior extremity of each bone is prolonged to the nostril by 

 a plate of cartilage. 



The vomer is a single bone placed in the median plane. It is 

 molded to form a groove for the reception of the septal cartilage 

 of the nose. 



The mandible, or lower jaw, is the largest bone of the face. It is 

 composed of a body and two branches. The body is the anterior 

 part, which lodges six incisor teeth. Behind these, in the male on 

 each side, is an alveolus for a canine tooth. The branches diverge 

 backward from the body to form the submaxillary space. Each 

 branch has a horizontal and a vertical part and presents two sur- 

 faces, two borders, and two extremities. Both surfaces of the 

 horizontal part are slightly convex and smooth. The superior 

 border has six alveoli for the lower molar teeth; the inferior border 

 is thick and rounded in the young animal, but becomes thin and 

 sharp in the old. The vertical part is wide and thin; the external 

 and internal surfaces are concave and rough and lodge the powerful 

 muscles of mastication. The anterior extremity joins the body, 

 while the posterior presents an articular condyle for the temporal 

 bone. A thin, flat process projects up from the front of this articu- 

 lar surface. 



The hyoid bone is situated between the vertical parts of the 

 mandible. It supports the root of the tongue, the pharynx, and 

 the larynx. 



Air-sinuses are spaces or cavities between the bones in the 

 head. They add to the volume of the head without increasing its 

 weight. In the fresh state they are lined with a continuation of 

 the mucous membrane from the nose, with which they communicate 

 by means of a small slit-like opening. There are four pairs of 

 sinuses, named the sphenoidal, frontal, superior maxillary, and 

 inferior maxillary. Of these, the superior maxillary sinus is the 

 largest and most important. It communicates freely with the 

 frontal sinus, but is separated from the inferior maxillary sinus 

 by a thin, imperforate plate of bone. The roots of one or more 

 teeth are seen in the sinus covered by a thin layer of bone. 



The vertebrae are divided into five regions or groups, designated 

 as cervical, thoracic, lumbar, sacral, coccygeal. Each group has a 

 typical number of segments as shown in the following formula: 



CvTisLeSsCy 15-21. 



