70 THE SKELETON OF THE HORSE 



The Vomer 



The vomer is a median bone, which assists in forming the ventral part of the 

 septum nasi. It is composed of a thin lamina which is bent (except in its posterior 

 part) so as to form a narrow groove (Sulcus vomeris) , in which the lower part of the 

 perpendicular plate of the ethmoid bone and the septal cartilage are received. The 

 lateral surfaces, right and left, are highest near the posterior end and diminish 

 gradually to the anterior end; they are slightly convex dorso-ventrally, and are 

 covered l3j' the nasal mucous membrane during life. The ventral border is thin 

 and free in its posterior third, and divides the choanae or posterior nares medi- 

 ally; in the remainder of its extent it is wider and is attached to the nasal crest. 

 The anterior extremity lies above the ends of the palatine processes of the premax- 

 illiE. The posterior extremity consists of two wings (Alae vomeris) which extend 

 outward below the body of the presphenoid; posteriorly they form a notch (In- 

 cisura vomeris), and laterally join the palatine and pterygoid bones. 



Development. — The vomer is primitively double, and ossifies from a center on 

 either side in the membrane covering the cartilaginous septum nasi; the two 

 laminae then fuse below and form a groove. 



The Mandible 



The mandible (AIandibula),i or lower jaw bone, is the largest bone of the 

 face. The two halves of which it consists at birth unite during the second or third 

 month, and it is usually described as a single bone. It carries the lower teeth, and 

 articulates Ijy its condyles with the squamous temporal on either side. It consists 

 of a body and two rami.^ 



The body (Corpus mandibulse) is the thick anterior part which bears the incisor 

 teeth. It presents two surfaces and a border. The lingual surface (Fades lin- 

 gualis) is smooth and slightly concave; during life it is covered by mucous mem- 

 brane, and the tip of the tongue overlies it. The mental surface (Facies mentalis) 

 is convex and is related to the lower lip. It is marked by a median furrow which 

 indicates the position of the primitive symphysis mandibulse. The curved alveolar 

 border (Limbus alveolaris) presents six alveoli for the incisor teeth, and a little 

 further back two alveoli for the canine teeth in the male; in the mare the latter are 

 usually absent or small. 



The rami (Rami mandibulse) extend backward from the body and diverge to 

 inclose the mandibular (or submaxillary) space (Spatium mandibulare) . Each 

 ramus is bent so as to consist of a horizontal part (Pars molaris) which bears the 

 lower cheek teeth, and a vertical part (Ramus mandibulse [S. N. A.]) which is ex- 

 panded and furnishes attachment to powerful muscles; the term angle is applied 

 to the most prominent part of the curve. The ramus presents two surfaces, two 

 borders, and two extremities. The lateral surface is smooth and slightly convex 

 from edge to edge on the horizontal part; at the junction with the body it presents 

 the mental foramen (Foramen mentale), which is the external opening of the man- 

 dibular canal. On the vertical part it is somewhat concave and presents a number 

 of rough lines for the attachment of the masseter muscle. The medial surface of 

 the horizontal part is smooth, and presents a shallow longitudinal depression in its 

 middle; above this there is often a faint mylo-hyoid line (Linea mylohyoidea) for 

 the attachment of the muscle of like name. At the lower part of the junction with 

 the body there is a small fossa for the attachment of the genio-hyoid and genio- 



' This bone is also commonly called the inferior maxilla. 



''- In the Stuttgart Anatomical Nomenclature (S. N. A.) the body (Corpus) is the part which 

 bears the teeth, and is divided into a pars incisiva and a pars molaris. The ramus is the rest of 

 the bone. This mode of description is copied from the Basel Nomina Anatomica. It does not 

 seem to the author to be well adapted to comparative purposes. 



