THE MANDIBLE. 



157 



before backwards, and bounds the mandibular notch posteriorly. To the medial side 

 of the neck, immediately below the condyle, there is a little depression (fovea ptery- 

 goidea) for the insertion of' the external pterygoid muscle. The convex surface of 

 the condyle is transversely 

 elongated, and so disposed 

 that its long axis is in- 

 clined nearly horizontally 

 medio - laterally and a 

 little forwards. The con- 

 vexity of the condyle is 

 more marked in its antero- 

 posterior than in its trans- 

 verse diameter, and tends 

 slightly to overhang the 

 mandibular notch. The 

 medial and lateral ends of 

 the condyle terminate in 

 tubercles which serve for 

 the attachment of part of 

 the articular capsule of 

 the joint. 



FIG. 166. DEVELOPMENT OF THE MANDIBLE. 



A, As seen from the medial side ; B, from the lateral side ; 

 C, showing accessory (metaplastic) cartilages (blue). 



(In A and B Meckel's cartilage is coloured blue.) 



Ossification. Its de- 

 velopment, is intimately as- 

 sociated with Meckel's carti- 

 lage, the cartilaginous bar of 

 the first visceral or man- 

 dibular arch. Meckel's car- 

 tilages, of which there are 

 two, are connected proxi- 

 mally with the periotic capsule and cranial base. These distal ends meet, but do, not 

 fuse, in the region of the symphysis. Ossification takes place chiefly from membrane, 

 in part from primordial cartilage (Meckel's cartilage), and also in part from accessory 

 (metaplastic) cartilages, which have no connexion with Meckel's cartilage, but arise 

 in the membrane from which the greater part of the bone is formed. Before ossifica- 

 tion commences three structures are seen lying side by side in the mandibular arch of the 

 . embryo. These are, from medial to lateral side, Meckel's cartilage, the inferior alveolar 

 nerve, which anteriorly divides into its two terminal branches, viz., the incisor and 

 mental nerves, and a dense connective tissue which stretches from before backwards 

 from close to the mid-line anteriorly to near the acoustic region posteriorly. Ossification 

 hi membrane commences about the fortieth to forty-fifth day in the angle between the 

 incisor and mental nerves ; it extends rapidly backwards under the mental nerve, which 

 grooves its upper surface, and is ultimately enclosed within the mental foramen. At the 

 same time the outer alveolar wall is formed by the extension of this ossifying membrane 

 bone, from which later, about the third month, is developed by backward growth the 

 angle and ramus, the latter surmounted by a well-defined coronoid process. About the 

 forty-fifth day the inner alveolar wall, the so-called splenial element, is formed by an 

 ingrowth from the anterior part of the floor of the mental groove. This passes below the 

 , incisor nerve and passes up between it and Meckel's cartilage, which it subsequently 

 ; overlaps, extending rapidly forwards and backwards to end posteriorly in the lingula 

 anterior to the point of origin of the mylo-hyoid nerve. The mandible, in point of time, 

 is the second bone to ossify, being preceded only by the clavicle. Ossification in 

 MeckeVs cartilage. This commences a little later than the first formation of the coronoid 

 process, opposite the first and second incisor tooth germs, not by independent ossification, 

 but by invasion of osteoblasts from the neighbouring membrane bone. The cartilage 

 becomes surrounded by shelves of bone projected medially both above and below it 

 from the main membrane bone. A bony tube is thus formed which extends from near 

 the mid -line anteriorly to the second milk tooth posteriorly. Within these limits 

 Meckel's cartilage becomes incorporated within the mandible. The extreme anterior 

 end of the cartilage does not, however, undergo ossification, and the posterior end, save 

 that part concerned in the formation of the malleus and incus, degenerates and ultimately 

 disappears. Ossification in accessory cartilages. These appear at the following sites : one, 



