Skull and Hyoid 



259 



rotate in the lower cavity, whereas the menisco-squamous joint is evidently only capable 

 of a sliding movement. 



A little consideration will show that it would be impossible to open the jaw wide 

 if it rotated on an axis going through the temporal articulation, for the parotid and 

 Sterno-mastoid would prevent any large movement of the angle backwards : this 

 difficulty is overcome by putting the axis of rotation further down, when the jaw 

 can be widely opened with half the 

 amount of movement backwards of 

 the angle but with a proportionate 

 movement forward of the upper end. 

 In such action the fixed point or axis 

 of rotation is at the attachment of 

 the long internal ligament : the 

 foramen for the entering nerve and 

 vessels is associated with this point, 

 so that they enter the bone at its 

 point of least movement. Some are 

 inclined to place the position of the 

 axis of movement lower still, at the 

 attachment of the stylo-mandibular 

 ligament. The External Pterygoid * 

 starts the movement of opening by 

 its contraction, thus drawing forward 

 the nbro-cartilage as well as the head 

 of the bone, so that the latter always 

 has a hollow cavity in which it can 

 turn : the forward movement of the 

 bone with its depression (caused by 

 its head being carried on to the 

 eminentia articularis) tends to tighten 

 the long ligament, and further action 

 of the outer Pterygoid can only pull 

 the upper end of the bone forward 

 on the axis passing through the 

 lingula. The long internal lateral 

 ligament is attached to the lingula 

 and the whole length of the adjoining 

 margin of the foramen ; it is fastened 

 above to the spine of the sphenoid, 

 which is on the inner border of the 

 glenoid cavity. The short internal 

 ligament is attached to the base of 



FIG. 209. Types of jaws from a child, an adult, and an old 

 person. The higher position of the mental foramen 

 in the child is owing to the imperfect development 

 of the alveolar part of the jaw, and in the old bone 

 to absorption of the alveolus following loss of teeth. 

 Notice also the difference between the angles ; the 

 angle becomes a necessity, as the mandible is depressed 

 in consequence of the increasing height of the maxilla, 

 for the purpose of keeping the margins of the two bones 

 parallel, and hence the sharpest angle goes with the full 

 dentition. 



the sphenoidal spine, and the line of 

 the rest of the capsule runs round the articular surface on the squama of the tem- 

 poral. The auriculo-temporal nerve runs back between these two ligaments (Fig. 182). 

 The condyle projects markedly inwards, and the neck exhibits a corresponding widen- 

 ing from side to side as it passes up to support the head : the wide part is concave 

 in front, gives attachment to the front part of the capsule and, below this, to the 



* The burden is thrown on this muscle only to show the movements of the joint ; it must be understood 

 that many other muscles are concerned. 



172 



