Motor System — Muscles and Skeleton 133 



and tarsal bones are arranged in a proximal row of three, a distal row 

 of five (each at the base of a digit), and one or two inserted between 

 the rows. 



For descriptive purposes, the limb is imagined to be extended in a 

 straight line perpendicular to the axis of the body, and without any 

 twisting among its parts. The pectoral limb being placed thus, the 

 radius and the pollex are on its anterior side and the palm of the 

 manus is ventral. Similarly, in the pelvic limb, the tibia and hallux 

 are anterior and the sole (plantar surface) is ventral. 



The skeletal parts of the tetrapod limb are accurately preformed 

 in cartilage, but in the adult, except tailed amphibians, they are 

 usually completely ossified. No membrane bones are added, except 

 in the pectoral girdle, but there are occasional local ossifications in 

 tendons forming so-called sesamoid bones. Such is the patella 

 ("kneecap"), common among mammals and found in some reptiles 

 and birds. Another sesamoid is the pisiform, a small bone commonly 

 found on the posterior edge of the carpus. 



In the relations of the girdles to the vertebral axial skeleton, 



there are significant differences. In most fishes the pectoral girdle is 

 attached to the axial skeleton but the pelvic girdle is never thus 

 attached. In modern tetrapods the pectoral girdle never has skeletal 

 connection with the vertebral axis, while the pelvic girdle (except 

 when the hindlegs are vestigial, as in whales) is always attached to 

 one or more sacral vertebrae. The attachment is by means of one or 

 more pairs of very short, stout ribs, each of which is strongly joined 

 distally to the upper end of the ilium and, proximally, usually fused 

 to the short, thick transverse process of its vertebra. 



In most fishes the essential propelling organ is the tail. The well- 

 developed pectoral fins serve mainly for balancing and steering. The 

 reduced condition of the pelvic fins shows that their locomotor function 

 is unimportant. Consistent with these facts, the pectoral girdle of 

 bony fishes is commonly attached, via the cleithrum and other mem- 

 brane bones, to the hind end of the cranium. In skates (broad, thin, 

 flat, but otherwise sharklike fishes: Fig. 122), the scapular process of 

 the girdle is greatly prolonged and joined to the vertebral column. 

 This exceptional condition is related to the fact that the skate's tail 

 is so reduced as to be useless for propulsion. Swimming is effected by 

 an undulatory flapping of the enormously expanded pectoral fins. 



In the locomotion of terrestrial tetrapods, the power is applied 

 at the rear — as in an automobile. The body is pushed forward by the 

 hindlegs. Mechanical efficiency — prevention of "lost motion" — re- 

 quires a rigid connection between the pelvic girdle and the main sup- 



