THE ARTICULATIONS OF THE SKULL— SYNARTHBOSES OF THE SKULL 217 



STERNAL ARTICULATIONS 



In the new-born foal the seven Ijony segments are united by persisting cartilage 

 (Synchondroses intersternales) . The last two segments coalesce within a few weeks 

 afterbirth. In old subjects there is more or less ossification of the intersternebral 

 cartilage, which may lead to fusion of adjacent segments, especially posteriorly. 

 The internal sternal ligament (Lig. sterni proprium internum) lies on the thoracic 

 surface of the sternum. It arises on the first segment, and divides opposite the 

 second chondro-sternal joint into three bands. The median band passes back- 

 ward and spreads out on the last segment and the xiphoid cartilage. The lateral 

 branches — thicker and wider — lie along the lateral borders above the chondro- 

 sternal joints, and end at the cartilage of the eighth rib; they are covered by the 

 transversus thoracis muscle. 



The Articulations of the Skull 

 mandibular articulation 



This joint (Articulatio mandibularis) is a diarthrosis formed between the ramus 

 of the mandible and the squamous temporal bone on either side. 



The articular surfaces are dissimilar in form and size. That on the squamous 

 temporal bone is concavo-convex, and the long axis is directed outward and some- 

 what forward; it consists of a condyle in front and a glenoid cavity, which is con- 

 tinued upon the postglenoid process behind. The mandible presents a trans- 

 versely elongated condyle. 



The articular disc (Discus articularis) is placed between the joint surfaces, 

 which it renders congruent. Its surfaces are molded upon the temporal and man- 

 dibular surfaces respectively, and its circumference is attached to the joint capsule; 

 thus it divides the joint cavity into upper and lower compartments, the former being 

 the more roomj^ 



The joint capsule is strong and tight. It is reinforced by two ligaments. The 

 lateral ligament (lig. lateralc) extends obliquely across the anterior part of the 

 lateral surface of the capsule, from which it is not distinctly separable. The pos- 

 terior ligament (Lig. posterius) is an elastic band which is attached above to the 

 postglenoid process, and below to a line on the posterior face of the neck of the 

 mandible. 



Movements. — The chief movements take place around a transverse axis pass- 

 ing through both joints. Associated with this hinge-like action is shght gliding 

 movement, as in opening and shutting the mouth. When the mouth is shut, 

 the condyle of the mandible lies under the glenoid cavity. When the mandible 

 is depressed, the condyle moves forward under the articular eminence of the tem- 

 poral bone, carrying the disc with it. In protrusion and retraction of the lower 

 jaw the gliding movement just described occurs without the hinge-like rotation 

 of the condyle. These movements are similar in both joints. In the transverse 

 movements (as usually performed in mastication) the action consists of rotation 

 of the condyles around a vertical axis, while the disc glides forward on one side and 

 backward on the other. 



SYNARTHROSES OF THE SKULL 



Most of the bones of the skull are united with the adjacent bones by sutures; 

 a few are united by cartilage. The difference in the uniting medium depends on 

 the fact that most of these bones are developed in membrane, but some are pre- 

 formed in cartilage. Most of these joints are temporary, and are obliterated at 



