Skull and Hyoid 261 



and backwards, so differing from those of the glenoid cavities ; they also differ in 

 that they are not directed downwards as well as inwards. Such differences 'n the 

 disposition of the bony surfaces can only be adjusted by the intervening meniscus. 

 The direction of the axis of the condyle inwards and backwards seems to be associated 

 with the side-to-side movement of the jaw ; when the jaw is moved, for instance, to 

 the right, the movement is accomplished by the left External Pterygoid and the right 

 Digastric, and the bone apparently rotates round a centre passing vertically through 

 the hyoid or behind this, the left condyle moving out and the right one in. From 

 this point of view the condyles might be considered to lie in the line of the periphery 

 of the circle in which the jaw moves : it is evident that they could not be directed 

 inwards and forwards, unless the centre of rotation lay far behind the jaw, and the 

 angle moved backwards. 



A large part of the body of the bone is accessible to digital examination. Thus prac- 

 tically the whole of the alveolus can be directly examined by the finger in the mouth : it 

 may be pointed out here that the line of the closed mouth is above the lower teeth and 

 its angle is about as far out as the first bicuspid. The outer aspect and lower border 

 of the splenial portion are not thickly covered and can be palpated, and the facial 

 artery can be compressed here against the bone. The inner aspect of this part is 

 inaccessible. 



The ramus, however, is thickly covered over nearly its whole extent, and can only 

 be palpated from the outside through Masseter and parotid, although the angle is fairly 

 clear. But the front border of the ramus can be investigated from the mouth : it can 

 be understood from Fig. 206 that the finger, passed back in the alveolo-labial sulcus or 

 along the cheek, can feel and examine the Temporal tendon and the ridges leading to 

 the coronoid process, through the plane of the Buccinator, and it is possible also to feel 

 the pterygo-mandibular ligament in this plane if the structure is put on the stretch by 

 opening the mouth. The student can familiarise himself with these observations by 

 examining his own mouth, and he will also be able to find his lingual nerve where it lies 

 on the bone under the mucous membrane behind and below the last molar. 



Development. 



The bone develops in the first or mandibular visceral arch from an ossification 

 commencing in membrane to the outer side of Meckel's cartilage during the sixth 

 week ; thus there is only one centre for each half of the bone. The centre first shows 

 in the lateral part of the bone, and, extending forward, grows round Meckel's cartilage 

 and encloses it in a groove which closes in later : it also spreads slowly upwards and 

 backwards to make the ramus and its processes. The condyle and coronoid are indi- 

 cated at the beginning of the third month and are practically formed by the middle 

 of this month : a cartilaginous change is visible in the mesenchyme of the condylar, 

 coronoid, and angular regions before the spreading ossification reaches them. 



The bone is in two halves at birth, and does not fuse until a year or more has elapsed. 



Meckel's cartilage atrophies in its greater part, but its front portion, taken into 

 the bone, is ossified, and probably therefore represented in the bone between the mental 

 foramen and the symphysis, and is possibly responsible for the prominence of the chin. 



THE DEVELOPMENT OF THE SKULL AS A WHOLE. 



The base of the skull is preformed in cartilage which begins to make its appearance 

 in the latter half of the second month : the vault is developed in membrane, as are the * 

 orbital plates of the frontal bone. 



