280 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



The sacral vertebrae are 4 in number, closely fused together. 

 The two first possess sacral ribs, which form lateral wing-like 

 expansions, articulating with the inner sides of the ilia. 



The caudal vertebrae, about 1 5 in number, gradually get smaller 

 towards the end of the tail, losing first their processes, and then 

 their neural arches. 



(c) The sternum is a narrow rod, made up of six laterally com- 

 pressed segments or sternelm, and placed in the mid-ventral line 

 of the thorax. The first segment (manubrium) is much larger 

 than the others, and possesses a prominent ventral ridge. It is 

 composed of two segments completely fused together, and this is 

 indicated in the adult by the attachment of the first sternal ribs 

 to the middle of its length. The remaining six pairs of sternal 

 ribs are connected with the junctions of the sternebrae, one pair 

 each to the first four, and two pairs to the last. The hindmost 

 sternebra is elongated, slender, and terminated by a rounded plate 

 of cartilage, with which it forms the xiphisternum. 



(2) Appendicular Endoskeleton. The long bones are terminated 

 as usual by epiphyses. As in other cases the skeleton of fore- or 

 hind-limb is divisible into girdle and free part. 



(a) Fore-limb (Fig. 84). The shoulder girdle is mainly composed 

 of the triangular scapula, placed external to the anterior ribs, 

 where it is held in place by muscles and ligaments. Its enlarged 

 apex presents a shallow glenoid cavity for the head of the humerus. 

 The base, anterior, and posterior sides of the triangle are termed 

 supra-scapular, coracoid, and glenoid borders respectively. With the 

 first a strip of cartilage, the supra-scapula, is connected. A con- 

 spicuous ridge, the spine, runs along the outer surface of the 

 scapula, and is produced below into a freely projecting process, 

 the acromion (ac), from which a more slender metacromion is 

 given off behind. The coracoid border is continued below into a 

 hook-like coracoid. process. This is originally distinct, and repre- 

 sents the coracoid bone of lower Vertebrates. A ligament, in 

 the centre of which is a slender curved clavicle, runs from the 

 tip of the manubrium to the acromion. To the sternal end of 

 this ligament a minute nodule of cartilage is attached which 

 represents a precoracoid. 



Free Limb. The humerus presents a proximal head, on its 

 upper (dorsal) surface, for articulation with the glenoid cavity, 

 and a distal pulley-like trochlea, which assists to form the elbow- 



