ENDOSKELETON AND VOLUNTARY MUSCLE 69 



vertebrates, though based on a common plan of structure, show 

 a wide variety of structural forms, in conformity with the fact 

 that they may be adapted for crawling, walking, running, 

 swimming, or flying. 



Forelimb of the Frog. — The fore- and hindlimbs of the frog 

 are made up of similar parts based on the same structural pattern. 

 The proximal element of the skeleton of the upper arm is an 

 elongated bone, the humerus, whose head articulates with the 

 glenoid cavity of the pectoral girdle to form the shoulder joint 

 (Fig. 40). The deltoid ridge is a narrow elevation running from 

 the head of the humerus to its middle. The lower end of the 

 humerus terminates in a large rounded articulating surface, which 

 fits into a depression in the olecranon of the forearm to form the 

 elbow joint. • The skeleton of the forearm is a single unit, the 

 radioulna, formed by the incomplete fusion of two bones, 

 the radius and ulna. The olecranon is a prolongation of the ulnar 

 portion of the radioulna and lies on the postaxial (away from the 

 head) side of the limb. The lower end of the radioulna is 

 definitely bifurcated into a preaxial radial component, in front, 

 and a postaxial ulnar component, behind. The bones of the 

 wrist or carpus consist of six small irregularly shaped bones, 

 known as car pals, arranged in two transverse rows. Beginning 

 on the preaxial side, the proximal row is made up of the centrale, 

 the radiale and, the ulnare; in the distal row are the first carpal, the 

 second carpal, and the outer carpal, the latter representing a 

 fusion of three bones. The skeleton of the palm or metacarpus, 

 which adjoins the wrist, is composed of five cylindrical bones of 

 which the preaxial one is greatly reduced in size, especially in the 

 female. Only four digits or fingers are present and these corre- 

 spond to digits II, III, IV, and V of the human hand, the thumb 

 (I) being absent. The bones of the fingers are called phalanges, of 

 which there are two in digits II and III and three in digits IV 

 and V. 



Hindlimb of the Frog. — The skeleton of the upper leg is the 

 femur, which corresponds with the humerus of the forelimb. It is 

 a cylindrical bone in the form of a slightly reversed curve. Its 

 rounded head fits into the acetabulum of the pelvic girdle to form 

 the hip joint. Its distal end is enlarged to form an articulating 

 surface with the tibiofibula of the lower leg or crus. The tibio- 

 fibula like the radioulna is a fusion of two bones. The double 



