THE SYNARTHROSES OF THE SKULL 179 



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

 dro-sternal joints, and end at the cartilage of the eighth ril); they are covered by 

 the transversus thoracis muscle. 



The Articulations of the Skull 

 temporo-mandibular articulation 



This joint (Articulatio numdibularis) is the only diarthrosis formed between 

 bones of the skull. 



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 glenoid cavity, which is continued upon the post- 

 glenoid process behind, and a condyle in front. The mandible presents a trans- 

 versely elongated condyle. 



The articular disc is placed between the joint surfaces, which it renders con- 

 gruent. Its upper and lower surfaces are molded upon the temporal and mandibular 

 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 roomy. 



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

 external ligament (Ligamentum laterale) extends oblicjuely across the anterior 

 part of the outer surface of the capsule, from which it is not distinctly separable. 

 The posterior ligament (Ligamentum posterius) is an elastic band which is attached 

 above to the ]wstglenoid 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 slight gliding 

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

 the condyle lies under the glenoid cavity. When the mandible is depressed, 

 the condyle moves forward under the articular eminence of the temporal 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 lateral 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. 



THE 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 

 various periods during development and growth. Their importance rests on the 

 fact that so long as they persist, continuous growth is possible. They are usually 

 designated according to the bones which enter into their formation, e. g., spheno- 

 squamous, naso-frontal, internasal, etc. Special names (borrowed from human 

 anatomy) are sometimes used; thus the interparietal, the parieto-occipital, and 

 the parieto-frontal sutures are often called the sagittal, lambdoid, and coronal re- 

 spectively. 



Detailed description of the sutures has not sufficient chnical value to justify much addition 

 to the statements'^made in the osteology in this connection. The obliteration or closure of the 

 sutures is, however, worthy of brief merition. The cranial sutures are usually all closed at seven 

 years, but the apex only of the petrous temporal is fused with the occipital and squamous temporal. 

 Most of the facial sutures are practically closed at ten years, although complete synostosis may in 



