68 



GENERAL ZOOLOGY 



is an unpaired structure consisting of the manubrium in front 

 (above), the gladiolus in the middle, and the ensiform process 

 behind (below). To the gladiolus are attached the intercostal 

 cartilages which link it with the ventral ends of the first ten pairs 

 of ribs. Its only contacts with the pectoral girdle are through 

 the clavicles and these are of minor importance as far as support- 

 ing the sternum is concerned (Fig. 39). 



Pelvic Girdle. — The pelvic girdle is much like the pectoral 

 girdle, with which it may be compared part for part. Thus, the 



pubis is on the ventral side in the same 

 relative position as the clavicle; the ischium, 

 lying posteriorly to the pubis, corresponds 

 to the coracoid; and the ilium, extending 

 dorsally, to the scapula (Fig. 41). The 

 pelvic girdle, however, differs from the 

 pectoral girdle in its relation to the axial 

 skeleton in all forms above the fishes. In 

 fishes, both girdles are free of the vertebral 

 Fig. 42.— Left human column, but in the higher groups the pelvic 

 scapula from the front. o;irdle is firmly united to the sacrum on each 



a, acromium process; c, ° . 



coracoid process; g, side by a sacroiliac ankylosis, a fusion ol the 

 glenoid cavity. sacrum and ilium. This, together with the 



fact that the pubic portions may meet in the midventral line, 

 makes the pelvis a firm, rigid structure. This is especially 

 important in bipeds like birds and man, where the entire weight 

 of the trunk rests upon the pelvis. In man, the three elements 

 of each side of the pelvis, though separate in the embryo, are 

 fused in the adult into a single bone, the os innominatum. 



Paired Appendages. — The anterior and posterior pairs of fins 

 of fishes are relatively simple structures, used in maintaining 

 balance and to a certain extent in swimming. They may be 

 classified as organs of locomotion, though the important strokes 

 in swimming are produced in the fish by the contractions of the 

 body muscles, which push the trunk and tail against the water. 

 The skeleton of the fin consists of cartilaginous or bony rods, 

 which support the tough membrane making up the rest of the 

 fin. Save for differences in details and some cases of special 

 adaptation, the paired fins are uniform in shape and structure, 

 in keeping with the uniform nature of the fluid medium, water, 

 in which they live. The paired appendages of the terrestrial 



