114 ESSENTIALS OF ZOOLOGY 



they are attached to the sternum by costal cartilages. The other 

 six pairs are false ribs because their costal cartilages do not join 

 the sternum. The eighth, ninth, and tenth pairs attach to the poste- 

 rior border of the seventh rib and by cartilage to the sternum. The 

 three posterior are free at the ventral ends and are known as floating 

 ribs. The sternum or breastbone consists of seven segments called ster- 

 nebrae, which join each other end to end. The most anterior one is 

 known as the manubrium and the seventh one, which terminates in 

 a rounded plate of cartilage, is the xiphoid process. 



The appendicular skeleton, consisting of the two girdles and pairs 

 of limbs, is quite well developed. The pectoral girdle is composed of 

 the shoulder blade or scapula which is a dorsally located, broad, flat, 

 more or less triangular bone, and the collar bone or clavicle, which 

 is a slender, double-curved rod of bone extending from the acromion 

 process on the apex of the scapula to the manubrium of the sternum. 

 At the level of the junction of the clavicle and scapula a socket is 

 formed for articulation with the bone of the upper arm. This is 

 the glenoid fossa. The bone of the upper arm is the humerus. On 

 its proximal head portion are the greater and lesser tuberosities which 

 are muscle attachments. The shaft is nearly cylindrical except in 

 the proximal portion where it is flattened to form the prominent 

 deltoid ridge on its anterior surface. The distal end is formed by a 

 medial head, the trochlea, and a lateral head, the capitulum. The 

 two bones of the forearm are the radius on the thumb side and the 

 ulna, parallel to it, on the opposite side. The radius articulates 

 with the capitulum of the humerus, and the ulna articulates with 

 the trochlea. The ulna is the larger and longer of the two bones. 

 Its projection beyond the articulation with the humerus forms the 

 elbow or olecranon process, and its projection at the articulation 

 with the wrist forms the styloid process. The radius is slender and 

 shorter. Its articulation with the capitulum forms a pivot and 

 allows some rotation of the forearm at the elbow. The hand is 

 composed of the nine car^pals of the wrist, arranged in two series; 

 the five metacarpals of the palm (the first is small) ; and the pha- 

 langes with three in each digit except the thumb which has only two. 

 The claw or nail is borne on the terminal phalanx. 



The pelvic girdle supports the hind limbs. This girdle is com- 

 posed of three pairs of bones. The three bones of each side are very 



