200 OSTEOLOGY 





in the lip (hare-lip). On examining a cleft palate in which the alveolus is not implicated, the cleft 

 will generally appear to be in the median line, but occasionally is unilateral and in some cases bilat- 

 eral. To understand this it must be borne in mind that three processes are concerned in the format ion 

 of the palate the palatine processes of the two maxillae, which grow in horizontally and unite 

 in the middle line, and the ethmovomerine process, which grows downward from the base of 

 the skull and frontonasal process to unite with the palatine processes in the middle line. In 

 those cases where the palatine processes fail to unite with each other and with the medial process, 

 the cleft of the palate is median; where one palatine process unites with the medial septum, the 

 other failing to do so, the cleft in the palate is unilateral. In some cases where the palatine pro- 

 cesses fail to meet in the middle, the ethmovomerine process grows downward between them and 

 thus produces a bilateral cleft. Occasionally there may be a hole in the middle line of the hard 

 palate, the anterior part of the hard and the soft palate being perfect; this is rare, because, as 

 a rule, the union of the various processes progresses from before backward, and therefore the 

 posterior part of the palate is more frequently defective than the anterior. 



THE EXTREMITIES. 



The bones by which the upper and lower limbs are attached to the trunk con- 

 stitute respectively the shoulder and pelvic girdles. The shoulder girdle or girdle 

 of the superior extremity is formed by the scapulae and clavicles, and is imperfect 

 in front and behind. In front, however, it is completed by the upper end of the 

 sternum, with which the medial ends of the clavicles articulate. Behind, it is 

 widely imperfect, the scapulae being connected to the trunk by muscles only. 

 The pelvic girdle or girdle of the inferior extremity is formed by the hip bones, 

 which articulate with each other in front, at the symphysis pubis. It is imperfect 

 behind, but the gap is filled in by the upper part of the sacrum. The pelvic girdle, 

 with the sacrum, is a complete ring, massive and comparatively rigid, in marked 

 contrast to the lightness and mobility of the shoulder girdle. 



THE BONES OF THE UPPER EXTREMITY (OSS A EXTREMITATIS SUPERIORIS). 



The Clavicle (Clavicula; Collar Bone). 



The clavicle (Figs. 200, 201) forms the anterior portion of the shoulder girdle. 

 It is a long bone, curved somewhat like the italic letter/, and placed nearly horizon- 

 tally at the upper and anterior part of the thorax, immediately above the first 

 rib. It articulates medially with the manubrium sterni, and laterally with the 

 acromion of the scapula. 1 It presents a double curvature, the convexity being 

 directed forward at the sternal end, and the concavity at the scapular end. Its 

 lateral third is flattened from above downward, while its medial two-thirds is of 

 a rounded or prismatic form. 



Lateral Third. The lateral third has two surfaces, an upper and a lower; and 

 two borders, an anterior and a posterior. 



Surface. The upper surface is flat, rough, and marked by impressions for the 

 attachments of the Deltoideus in front, and the Trapezius behind; between these 

 impressions a small portion of the bone is subcutaneous. The under surface is 

 flat. At its posterior border, near the point where the prismatic joins with the 

 flattened portion, is a rough eminence, the coracoid tuberosity (conoid tubercle}; 

 this, in the natural position of the bone, surmounts the coracoid process of the 

 scapula, and gives attachment to the conoid ligament. From this tuberosity an 

 oblique ridge, the oblique or trapezoid ridge, runs forward and lateralward, and 

 afford attachment to the trapezoid ligament. 



1 The clavicle acts especially as a fulcrum to enable the muscles to give lateral motion to the arm. It is accordingly 

 absent in those animals whose fore-limbs are used only for progression, but is present for the most part in animals 

 whose anterior extremities are clawed and used for prehension, though in some of them as, for instance, in a large 

 number of the carnivora it is merely a rudimentary bone suspended among the muscles, and not articulating with 

 either the scapula or sternum. 



