180 THE ARTICULATIONS OF THE HORSE 



some be delayed for years or may not occur at all; the nasal suture, for example, often persists 

 even in advanced age, so far as its anterior part is concerned. 



The principal synchondroses are: (1) That between the basioccipital and the 

 body of the sphenoid (Synchondrosis sphenooccipitahs) ; (2) that between the 

 presphenoid and postsphenoid (Synchondrosis intersphenoidahs) ; (3) those 

 between the parts of the occipital bone (Synchondroses intraoccipitales). The first 

 is usually ossified at four years, the second at three years, and the occipital bone is 

 consolidated at two years. 



The symphysis of the lower jaw (Symphysis mandibulae) ossifies at one to six 

 months. 



THE HYOIDEAN ARTICULATIONS 



The temporo-hyoid articulation is an amphiarthrosis, in which the dorsal angle 

 of the proximal end of the great cornu (Stylo-hyal) is attached by a short bar of 

 cartilage to the hyoid process of the petrous temporal bone. The cartilage (Arthro- 

 hyal) is about half an inch (ca. 1 to 1.5 cm.) in length. The chief movement is hinge- 

 like, the axis of motion passing transversely through both joints. 



The intercomual articulation is an amphiarthrosis formed by the junction of 

 the distal extremity of the great cornu with the proximal end of the small cornu 

 (kerato-hyal) . They are united by a very short piece of cartilage, in which there 

 is usually a small nodule of bone in the young subject. This nodule, the epihyal 

 or middle cornu, is usually fused with the great cornu in the adult. The chief 

 movement here is also hinge-like, the angle between the cornua being increased or 

 diminished. 



The basi-comual articulation is a diarthrosis formed by the junction of each 

 small cornu (kerato-hyal) with the body (basi-hyal). The small cornu has a con- 

 cave facet which articulates with the convex facet on either end of the dorsal sur- 

 face of the body. The capsule is ample enough to allow considerable movement, 

 which is chiefly hinge-like. The movements of the hyoid bone are concerned 

 chiefly in the acts of mastication and swallowing. In the latter the distal parts of 

 the hyoid bone are moved forward and upward, carrying the root of the tongue and 

 the larynx with them, and then return to their former position. 



The Articulations of the Thoracic Limb 



In the absence of the clavicle the thoracic limb forms no articulation with the 

 trunk, unless we regard as such the union by muscles. The movement of the 

 shoulder on the chest-wall is chiefly rotation around a transverse axis passing 

 through the scapula behind the upper part of the spine. 



THE SHOULDER JOINT 



The shoulder or scapulo-humeral joint (Articulatio scapulo-humeralis) is 

 formed by the junction of the distal end of the scapula with the proximal end of 

 the humerus. The articular surfaces are: (1) On the scapula, the glenoid cavity; 

 (2) on the humerus, the head. Both surfaces are approximately spherical and 

 similar in curvature, ])ut the humeral surface is much more extensive than that of 

 the scapula. 



The joint capsule is ample enough to allow the bones to be drawn apart about 

 an inch (ca. 2 to 3 cm.) ; l)ut this requires a very con.siderable amount of force unless 

 air is admitted into the joint cavity. The fibrous layer (or capsular ligament) is 

 not attached to the margin of the joint surfaces, but at a distance of one to two 

 centimeters from it. It is strengthened in front by two diverging elastic bands, 



i 



