134 Basic Structure of Vertebrates 



porting axis of the body. The action of the forelegs exerts upon the 

 body more "pull" than "push." For transmission of a pull, the tense 

 muscles connecting the pectoral girdle with the vertebral column and 

 ribs are quite adequate and, on occasion, the absence of a rigid con- 

 nection between girdle and vertebral column is a great advantage. 

 During such actions as leaping and galloping, the forelegs sustain 

 great impact. The cushioning of the girdle in a bulky mass of shoulder 

 muscles provides an excellent "shock absorber" — and even better 

 when, as in the cat, the clavicle is much reduced and only loosely tied 

 to the sternum ; or, as in the horse, the clavicle is lacking, leaving the 

 girdle without rigid connection to the sternum. 



The basic plan of the tetrapod appendage has been described. The 

 "pattern," however, is subjected to innumerable functional adapta- 

 tions, depending on the mode of life of the animal. The mechanical 

 demands imposed by swift running, climbing, swimming, digging, 

 flying, or grasping must be met by appropriate anatomic adjustments 

 which may involve changes in relative size and form of skeletal parts, 

 and reduction by omission of parts or by fusion of adjacent parts (see 

 Figs. 295, 441, 599). To a great extent, these changes in the skeleton 

 merely reflect necessary modifications of the muscular mechanism of 

 the appendage. 



MEDIAN APPENDAGES 



Median or unpaired appendages exist in most wholly aquatic 

 vertebrates. Fishes commonly have dorsal and ventral median fins 

 varying in number and position, and the tail tapers back into the 

 base of the bladelike vertical caudal fin (Fig. 127). The dorsal and 

 ventral fins are supported by basal "bars" and "rays," either carti- 

 laginous or bony (Fig. 128). The thin distal part of the fin is usually 

 stiffened by slender rays of a dermal hornlike substance. The basal 

 structures of dorsal fins may lie between successive neural spines of 

 vertebrae or may be tied to the spines so that the fin is anchored to the 

 vertebral column. The caudal fin may receive its chief support from 

 the greatly enlarged and laterally flattened neural and hemal spines 

 of several vertebrae at the base of the fin (Fig. 129). 



A median caudal fin occurs in amphibian tadpoles and in adult 

 aquatic salamanders but has no skeletal support other than the noto- 

 chord or vertebral column which forms its axis. The dorsal median fin 

 of many of the whalelike mammals contains no special skeleton. The 

 caudal fin of whales ("tail-flukes") extends in a horizontal plane and 

 has no skeleton other than the tip of the vertebral column. 



