PHYSICAL SUPPORT AND MOVEMENT 



93 



according to the same plan and may be compared bone for bone, humerus 

 with femur, radius and ulna with tibia and fibula, respectively, carpal 

 (wrist) bones with tarsal (ankle) bones, metacarpals with metatarsals 

 (body of hand and foot, respectively) and phalanges (bones of the digits) 

 of the hand with those of the foot. Vertebrates with primitive limbs have 

 five digits on fore and hind feet, but the limbs of specialized animals 

 have undergone more or less extensive modifications from the original 

 five-fingered and five-toed plan. In them usually the number of digits 

 has been reduced. 





int 



a 



cn.Z 



mtts.tjj 



,»/ 



mtts.S 



Ein 



Fig. 82. — Diagrams of generalized fore {A) and hind (B) limbs with limb girdles: 

 acth, acetabulum; CL, clavicle; en. 1, en. 2, centralia; COR, coracoid; dst. 1-5, distal row 

 of carpals and tarsals; FE, femur; FI, fibula; fi, fibulare; gl, glenoid fossa; I-V, digits; 

 HU, humerus; IL, ilium; int, intermedium; IS, ischium; mtcp. 1-5, metacarpals; mtts. 1-5, 

 metatarsals; ph, phalanges; p.cor, precoracoid; PU, pubis; RA, radius; ra, radiale; SCP, 

 scapula; TI, tibia; ti, tibiale; UL, ulna; ul, ulnare. (From Parker and Haswell, "Textbook 

 of Zoology.") 



The Motive Power. — The movement of structures in the higher 

 animals, whether these structures contain parts of the skeleton or not, 

 is all effected by muscles. Protoplasm in general has the power of 

 contracting, and in the protozoa there are motile structures, the cilia and 

 flagella, which have already been described (page 51). The muscles are, 

 however, much more specialized than any of these. 



In general, the muscles are arranged in opposing pairs or sets. In 

 the earthworm, in which crawling is effected by alternate contraction 

 and expansion of the length of the animal, there is one set of muscles 

 running lengthwise, another passing circularly around the body. With 

 the front end of the worm holding to the soil with its sloping setae, a wave 

 of contraction of the lengthwise muscles draws up the rest of the body. 



