ARTICULATIONS OF THE SKULL 215 



case of the hand, the line to or from which adduction and abduction are made is drawn through 

 the middle fin.ger, while in the foot it is through the second toe. 



Rotation is the revolution of a bone about its own axis without much change of position. 

 It is onl}' seen in enarthrodial and trochoidal joints. The knee also permits of slight rotation in 

 certain positions, which is a distinctive feature of this articulation. 



Circumduction is the movement compounded of the four angular movements in quick 

 succession, by which the moving bone describes a cone, the proximal end of the bone forming the 

 apex, while the distal end describes the base of the cone. It is seen in the hip and shoulder, as 

 well as in the carpo-metacarpal joint of the thumb, which thus approximates to the ball-and- 

 socket joint. 



In some situations where a variety of motion is required, strength, security, and celerity 

 are obtained by the combination of two or more joints, each allowing a different class of action, 

 as in the case of the wrist, the ankle, and the head with the spine. Many of the long muscles, 

 which pass over two or more joints, act on all, so tending to co-ordinate their movements and 

 enabUng them to be produced with the least expenditure of power. Muscles also act as elastic 

 ligaments to the joints; and when acting as such, are diffusers and combiners, not producers 

 of movement; the short muscles producing movement, the long diffusing it, and thus allowing 

 the short muscles to act on more than one joint. 



Muscles are so disposed at their attachments near the joints as never to strain the hga- 

 ments by tending to pull the bones apart, but, on the contrary, they add to the security of the 

 joint by bracing the bones firmly together during their action. 



The articulations may be divided for convenience of description into those: 1. 

 of the Skull; 2. of the Trunk; 3. of the Upper Limb; and 4. of the Lower Limb. 



THE ARTICULATIONS OF THE SKULL 



The movable articulations of the skull comprise (1) the mandibular; and (2) 

 those between the skull and the vertebral column, namely (a) between the occiput 

 and atlas; (6) between the atlas and epistropheus (axis); and (c) the ligaments 

 which connect the occiput and epistropheus. 



The union of the atlas and epistropheus is described in this section because, 

 (1) there is often a direct communication between the synovial cavity of the trans- 

 verse epistrophic and the occipito-atlantal joints; (2) the rotatory movements of 

 the head take place around the dens (odontoid process) ; and (3) important liga- 

 ments from the dens pass over the atlas to the occiput. 



(1) THE MANDIBULAR ARTICULATION 



Class. — Diarthrosis. Subdivision. — Condylarthrosis. 



The parts entering into the formation of this joint (figs. 256, 257) are: — the 

 anterior portion of the mandibular fossa and glenoid ridge (eminentia articularis) 

 of the temporal bone above, and the condyle of the lower jaw below. Both are 

 covered with articular cartilage, which extends over the front of the glenoid ridge 

 to facilitate the play of the interarticular cartilage. The ligaments which unite 

 the bones are: 



1. Articular capsule. 3. Spheno-mandibular. 



2. Articular disc. 4. Stylo-mandibular. 



The articular capsule is often described as consisting of four portions, anterior, 

 posterior, lateral and medial, which are, however, continuous with one another 

 around the articulation. 



1. The anterior portion consists of a few stray fibres connected with the anterior margin 

 of the articular disc, and attached below to the anterior edge of the condyle, and above to the 

 front of the articular eminence. Some fibres of insertion of the external pterygoid pass between 

 them to be inserted into the margin of th*^ articular disc. 



2. The posterior portion is attached above, just in front of the petro-tympanic (Glaserian) 

 fissure, and is inserted into the back of the jaw just below its neck. 



13. The lateral portion or temporo -mandibular (external lateral) ligament (fig. 256) is the 

 strongest part of the capsule. It is broader above, where it is attached to the lower edge of 

 the zygoma in nearly its whole length, as well as to the tubercle at the point where the two 

 roots of the zygoma meet. It is incUned downward and backward, to be inserted into the 

 , condyle and neck of the mandible laterally. Its fibres diminish in obliquity and strength from 

 before backward, those coming from the tubercle being short and nearly vertical. 



4. The medial portion (or short internal lateral ligament) (fig. 257) consists of well-defined 

 fibres, having a broad attachment, above to the lateral side of the spine of the sphenoid and 

 medial edge of the mandibular fossa; and below, a narrow insertion to the medial side of the neck 



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