THE JOINTS OF THE THORAX. 261 
relation of the condyle to the articular eminence occurs during the exaggerated depression of 
the mandible which results from yawning, in which position the articulation is liable to be 
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dislocated. When the two joints perform the same movement alternately, a certain amount of 
ateral motion results, from the fact that the long axis of each joint presents a slight obliquity 
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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 external lateral ligament which becomes tense. 
In all movements of the mandible the meniscus conforms closely to the position of the con- 
dyle, and they move forwards and backwards together, but at the same time the meniscus does 
not restrict the movements of the condyle. Thus while the meniscus, alone with the condyle, is 
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gliding upon the temporal aspect of the joint, the condyle itself revolves upon the inferior 
surface of the meniscus. 
CRANIAL LIGAMENTS NOT DIRECTLY ASSOCIATED WITH ARTICULATIONS. 
The stylo-mandibular ligament (lig. stylo-mandibulare, Figs. 204 and 206) 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. 
The pterygo-spinous ligament (lig. pterygo-spinosum) is a membrane extending 
from the upper part of the posterior free margin of the external pterygoid plate, 
backwards and slightly outwards, to the spinous process 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. 
The stylo-hyoid ligament (lig. stylo-hyoideum), 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. 
Costo-vertebral articulations (articulationes costo-vertebrales). 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 vertebrz, 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. 
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Costo-CENTRAL JOINTS. 
The articulations of the heads of the ribs with the centra or bodies of 
the vertebree (articulationes capitulorum, Fig. 199) are all diarthroses, which, from 
their somewhat hinge-like action, may be classed as ginglymoid. 
The head of every typical rib is wedge-shaped, and presents two articular 
facets, an upper and a lower, separated from each other by an antero-posterior 
ridge which abuts against an intervertebral disc, while the articular facets articu- 
late with similar surfaces on the contiguous margins of the two vertebra adjoining 
the disc. These surfaces form a wedge-shaped depression or cup, the bottom of 
which is more elastic than the sides, and thus an arrangement is provided which 
tends to reduce the shock of blows upon the walls of the chest. 
Each of these articulations is provided with a capsular ligament which 
surrounds and encloses the joint, and is attached to contiguous non-articular 
margins on the head of the rib and the two vertebral bodies. On its anterior or 
ventral aspect the capsule presents three radiating fasciculi which collectively form 
the stellate or anterior costo-vertebral ligament (lig. capituli costz radiatum, Fig. 
199). These fasciculi radiate from a centre on the front of the head of the rib, so 
that the middle fasciculus becomes attached to the intervertebral disc, while 
a 
