284 INEZ L. WHIPPLE. 



tion during active swimming involve a constant exhibition of cor- 

 responding changes of contour ; the constriction of the posterior 

 abdominal region occurs when the swimming motion is upward 

 (Fig. 10, a], the prominent bulging when the motion is down- 

 ward. Further, a sudden change from a downward to an upward 

 direction is preceded by an exaggerated constriction of the pos- 

 terior part of the cavity, an act which conspicuously involves the 

 sudden vigorous inpulling of the ypsiloid region. Evidently there 

 is in operation some mechanism for controlling tlie direction of tJie 

 body, ivhet/icr at rest or in motion, through the control of the rela- 

 tive buoyancy of anterior and posterior ends. 



C. The Hydrostatic Mechanism of Lunged Salamanders. 



Turning now to the anatomy of the ypsiloid region, the 

 explanation of this hydrostatic mechanism becomes very simple. 

 The contraction of the M. ypsiloideus posterior exerts a strong 

 pull upon the whole ypsiloid cartilage. The origin and insertion 

 of this muscle are, however, so nearly in the same plane as the 

 fulcrum (the articulation of the ypsiloid cartilage with the pubis) 

 that it seems at first a question as to whether the cartilage would 

 be bent upward or downward (/. e., dorsally or ventrally) by the 

 contraction of this muscle alone. It must be remembered, how- 

 ever, that the muscle is inserted into the upper (dorsal) surface 

 of the stem of the cartilage and also that the more strongly 

 developed portion of the muscle has its origin in the lateral por- 

 tion of the pubis, a region which owing to the convexity of the 

 body is slightly higher (more dorsal) than the insertion of the 

 muscle and the articulation of the cartilage. Moreover, outside 

 of the whole apparatus there are muscular walls (external oblique 

 and rectus abdominis) which would resist any tendency to bend 

 the cartilage downward, and with origin and insertion on so nearly 

 the same plane as the fulcrum it requires only a slight resistance 

 of this sort to turn the scale. Other muscles, moreover, are 

 attached to the ypsiloid cartilage and cooperate with the ypsi- 

 loideus posterior to determine the direction of motion. The con- 

 traction of the ypsiloid portion of the transversalis exerts a decided 

 upward (dorsalward) pull upon the ypsiloid cartilage while the 

 anterior ypsiloid muscle, pulling upon the arms of the cartilage 



