aiTGLOGY OF THE OENITHORHYNCHUS. 147 



in the act of giving the stroke to propel the body through the water, 

 are so inserted as to have a strongly rotating effect upon the bone, 

 twisting the limb in such a manner as to throw the elbow up and 

 away from the body. Now the two bones of the forearm are in mutual 

 apposition for their whole length, and so closely bound, that relative 

 motion upon each other is abrogated ; the forearm is confined perma- 

 nently in a state of semi-pronation. But it is evident that the broad 

 surface of the webbed hand must strike the water in the backward 

 stroke, and that the thin edge of the palm must cleave the water in 

 the bringing forward of the member ; that is, there must be perfect 

 virtual pronation and suj)ination during each stroke and return to 

 position for the next one. With the forearm bones stiffly bound to- 

 gether to ensure strength and fixity of tlie wrist and hand, this requi- 

 site rotation of the forearm and change through 180° of the plane of 

 the webs is efiected by the construction of the elbow-joint, and dispo- 

 sition of the muscles that act from the humerus and scapular arch 

 upon the proximal end of the forearm. The elbow, instead of being 

 the most strict ginglymus in the body, as usual among mammals, is 

 largely amphiarthrodial, permitting free rotation or lateral rocking of 

 the forearm upon the humerus. This compound motion seems to be 

 very nearly like what would occur as a i-esultant if, in man for in- 

 stance, the rotation of the head of the radius in its ulnar socket 

 should enter as a component of, and be merged into, the to-and-fro 

 swinging of the forearm upon the humerus. In a word, the animal 

 feathers its oars at the elhow-joint — not at the wrist. 



In studying the action of the. several muscles concerned, we see 

 clearly how this is efl'ected and find the reason for certain peculiari- 

 ties in their disposition. We have only to add further, in this con- 

 nection, that the articular facet of the humerus is not directly at the 

 end of the main axis of the bone, but displaced to one side, so as to 

 be at the base of the immensely developed ectocondylar process ; 

 that both the widely divaricating condylar processes ofler salient 

 points-d'appui for the muscular tractions that rotate the forearm ; and 

 that the olecranon is a very broad plate curving far up behind the 

 humerus, with widely expanded corners, in subserviency to the varied 

 action of different parts of the triceps and anconaeal muscles. 



(a'. — Prom the body.) 



Dorso-epitrochlearis. — The forearm slip from the latissimus is well 

 developed. It is given ofi' obliquely from the lower border of the 

 muscle, a little more than an inch from its humeral insertion, and 

 mounts upon the back of the forearm, crossing the limb over the 

 most prominent ridge of the latter. It appears to end in fascia over 

 the middle of the back of the forearm ; but may be traced, without 

 unduly forcing the dissection, to pretty definite insertion into the 



