218 THE ARTICULATIONS 



the vertical axis of its neck in the lower compartment, while the cartilage glides obliquely for- 

 ward and inward on one side, and backward and inward on the other, upon the articular surface 

 of the squamosal bones, each side acting alternately. If the symphysis be simply moved from 

 the centre to one side and back again, and not from side to side as in grinding, the condyle of 

 that side moves round the vertical axis of its neck, and the opposite condyle and cartilage glide 

 forward and inward upon the mandibular fossa. But in the ordinary grinding movement, one 

 condyle advances and the other recedes, and then the first recedes while the other advances, 

 slight rotation taking place in each joint meanwhile. 



Relations. — The chief relations are: Behind, and overlapping the lateral side, the parotid 

 gland. Laterally, the superficial temporal artery. Medially, the internal maxillary artery 

 and auriculo-temporal nerve. In front, the nerve to the masseter muscle. 



Muscles acting on the joint. — Elevators of the mandible. — temporals, masseters, int. 

 pterygoids. 



Depressors. — Mylo-hyoids, digastrics, genio-hyoid, muscles connecting the hyoid bone to 

 lower points. Ext. pterygoids. The weight of the jaw. 



Protractors. — Ext. pterygoids, superficial layer of masseters, anterior fibres of temporals. 



Retractors. — Posterior fibres of temporals, slightly by the int. pterygoids and deep layer of 

 the masseters. 



(2) THE LIGAMENTS AND JOINTS BETWEEN THE SKULL AND 

 VERTEBRAL COLUMN, AND BETWEEN THE ATLAS AND 



EPISTROPHEUS 



(a) The Articulation of the Atlas with the Occiput 



Class. — Diarthrosis. Subdivision. — Double Condylarthrosis. 



This articulation [articulatio atlanto-occipitalis] consists of a pair of joints 

 symmetrically situated on either side of the middle line. The parts entering into 

 their formation are the cup-shaped superior articular processes of the atlas and 

 the condyles of the occipital bone. They are united by the following ligaments :— 



1. Anterior atlanto-occipital. 3. Two articular capsules. 



2. Posterior atlanto-occipital. 4. Two anterior oblique. 



The anterior atlanto-occipital ligament [membrana atlanto-occipitalis anterior] 

 (fig. 259) is less than an inch (about 2 cm.) wide, and is composed of densely 

 woven.fibres, most of which radiate slightly lateralward as they ascend from the 

 ^ront surface and upper margin of the anterior arch of the atlas to the anterior 

 border of the foramen magnum; it is continuous at the sides with the articular 

 capsules, the fibres of which overlap its edges, and take an opposite direction 

 mediall}^ and upward. 



The central fibres ascend vertically from the anterior tubercle of the atlas to the pharyn- 

 geal tubercle on the occipital bone; they are thicker than the lateral fibres, and are continuous 

 below with the superficial part of the anterior atlanto-epistrophic ligament, and through it 

 with the anterior longitudinal ligament of the vertebral column. It is in relation, in front, 

 with the recti capitis anteriores; and behind, with the apical dental or suspensory ligament. 



The posterior atlanto-occipital ligament (fig. 260) is broader, more mem- 

 branous, and nqtso strong as .the anterior. It extends from the posterior surface 

 and upper border of the posterior arch of the atlas to the posterior margin of the 

 foramen magnum from condyle to condyle; being incomplete on either side for the 

 passage of the vertebral artery into, and suboccipital nerve out of, the canal. It is 

 somewhat thickened in the middle line by fibres, which pass from the posterior 

 tubercle of the atlas to the lower end of the occipital crest. 



It is not tightly stretched between the bones, nor does it limit their movements; it corre- 

 sponds with the position of the ligamenta flava, but ha,{LJio elastic tissue in its composition. 

 It is in relation in front with the dura mater, which is firmly attached to it; and behind with the 

 recti capitis posteriores minores, and enters into the floor of the suboccipital triangle. Its 

 lateral margins, which do not reach the occipital bone but terminate on the posterior end of 

 the superior articular processes of the atlas, form the so-called oblique ligaments of the atlas. 

 The lateral margins of these ligaments are free and they form the posterior boundaries of the 

 aj)crture.s through which the vertebral arteries enter and the suboccipital nerves leave the 

 vertebral canal. 



The atlanto-occipital articular capsules (figs. 259 and 260) are very distinct 

 and strongly marked, except on the medial side, where they are thin and formed 

 only of short membranous fibres. They are lax, and do not add much to the 

 security of the joint. 



