the vomer — the mandible 63 



The Vomer 



The vomer is a median unpaired bone, which assists in forming the lower 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 septi narium) , in w^hich the 

 lower part of the perptnidicular ])late of the ethmoid ]:)one 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 by the nasal mucous membrane during life. The in- 

 ferior border is thin and free in its posterior third, and divides the posterior nares 

 into right and left halves; in the remaind(n- of its extent it is wider and is attached 

 to the nasal crest. The anterior extremity lies above the ends of the palatine pro- 

 cesses of the premaxillie. The posterior extremity consists of two wings (Alse 

 vomeris) which extend outward below the body of the pre-sphenoid ; posteriorly 

 they form a notch (Incisura vomeris), and laterally join the palate 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 laminai 

 then fuse below and form a groove. 



The Mandible 



The mandible or inferior maxilla (Alandibula) is the largest and the only 

 movable bone of the face. The two halves of which it consists at birth unite during 

 the second or third month, and it is usually descril)ed as a single bone. It carries 

 the lower teeth, and articulates by its condyles with the squamous temporal on 

 either side. It consists of a body and two rami.^ 



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

 teeth. It presents two surfaces and a border. The lingual or superior surface 

 (Facies lingualis) is smooth and slightly concave; during life it is covered by mucous 

 membrane, and the tip of the tongue overlies it. The labial or 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 

 curvetl 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 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) which is expanded 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 external surface is smooth and slightly convex from (>dge 

 to edge on the horizontal part; at the junction with the l^ody it presents the mental 

 foramen (Foramen mentale), which is the external opening of the mandibular or 

 inferior dental canal. On the vertical part it is somewhat concave and presents a 

 number of rough lines for the attachment of the masseter muscle. The internal 

 surface of the horizontal part is smooth, and presents a shallow longitudinal de- 

 pression in its middle; above this there is often a faint mylo-hyoid line for the at- 

 tachment 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-glossus 

 muscles. On the vertical part the surface is concave, and is marked in its lower and 



' In the Stuttgart Nomenclator Anatomicus (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. 



