THE JOINTS OF THE THORAX. 313 



Portions of the following structures are found in the interval between the spheno-mandibular 

 ligament and the ramus of the mandible viz., the external pterygoid muscle; internal 

 maxillary vessels ; inferior alveolar vessels and nerve ; middle meningeal vessels ; auriculo- 

 temporal nerve ; and sometimes a deep portion of the parotid gland. 



Movements of the Mandible. The nature of the movements which the mandible can 

 perform is determined partly by the character of the articular surfaces of the mandibular joint, 

 and partly by the fact that, while the two joints always act simultaneously, they may also, 

 to some extent, perform the same movement alternately. 



When movement takes place through the long or transverse horizontal axis of each joint, 

 the mandible may be elevated, as in clenching the teeth, or it may be depressed, as in gaping. 

 In the latter movement the condyle leaves the mandibular fossa, and, along with the disc, it 

 moves forwards until they rest upon the tuberculum articulare. Meantime the chin describes 

 the arc of a circle, of which the centre or point of least movement corresponds to the position of 

 the inferior alveolar foramen, and thus the structures which enter at that foramen are protected 

 against stretching. Coincidently with the forward movement of the condyle, it glides in a 

 revolving manner upon the inferior aspect of the disc. 



At any stage in the movement of depressing the chin the mandible may be protruded, so 

 that the inferior incisor teeth are projected in front of the'upper set, a movement which results 

 from the condyles of the mandible being drawn forwards upon the articular tubercles. A similar 

 relation of the condyle to the articular tubercle occurs during the exaggerated depression of 

 the mandible which results from yawning, in which position the articulation is liable to be 

 dislocated. When the two joints perform the same movement alternately, a certain amount of 

 lateral motion results, from the fact that the long axis of each joint presents a slight obliquity 

 to the transverse axis of the skull, and consequently a grinding or oblique movement in the 

 horizontal plane is produced. Excessive depression, with the risk of dislocation, is resisted by 

 the fibres of the temporo-mandibular ligament, which becomes tense. 



In all movements of the mandible the disc conforms closely to the position of the condyle, 

 and they move forwards and backwards together, but at the same time the disc does not restrict 

 the movements of the condyle. Thus while the disc, along with the condyle, is gliding upon 

 the temporal aspect of the joint, the condyle itself revolves upon the inferior surface of the disc. 



CRANIAL LIGAMENTS NOT DIRECTLY ASSOCIATED WITH ARTICULATIONS. 



Lig. Stylomandibulare. The stylo-mandibular ligament (Figs. 298 and 300) is a 

 specialised portion of the deep cervical fascia which extends from the anterior aspect 

 of the tip of the styloid process of the temporal bone to the- posterior border of the 

 angle of the mandible, between the insertions of the masseter and internal pterygoid 

 muscles. 



Lig. Pterygospinosum. The pterygo-spinous ligament is a membrane extending 

 from the upper part of the posterior free margin of the lateral pterygoid lamina, 

 posteriorly and slightly laterally, to the angular spine of the sphenoid. An 

 interval is left between its upper border and the floor of the skull for the outward 

 passage of those branches of the inferior maxillary nerve which supply the 

 external pterygoid, temporal, and masseter muscles. This ligament has a tendency 

 to ossify either wholly or partially. 



Lig. Stylohyoideum. The stylo-hyoid ligament may be regarded as the down- 

 ward continuation of the styloid process of the temporal bone. Inferiorly it is 

 attached to the lesser cornu of the hyoid bone. It is not infrequently ossified, in 

 which case it constitutes the epihyal bone found in many animals. 





THE JOINTS OF THE THORAX. 



Articulationes Costovertebrales (Costo - vertebral Articulations). - - The 



typical rib articulates with the vertebral column both by its head and by its 

 tubercle. Thus, two sets of articulations, with their associated ligaments, exist 

 between the ribs and the vertebrae, but each set is constructed upon a common 

 plan, with the exception of certain joints situated at the upper and lower ends of 

 the series, where the ribs themselves deviate from the typical form. 



ARTICULATIONES CAPITULORUM. 



The articulations of the heads of the ribs with the bodies of the vertebras 

 (Fig. 293) are all diarthroses, which, from their somewhat hinge-like action, may 

 be classed as ginglymoid. 



