THE PECTORAL LIMB 



flexed; and the third extension: or by operating the first in antagonism 

 to the second, all three can act in extension. And extension of the flipper 

 is very important in the act of drawing herbage toward the mouth. In 

 the manati the deltoid is inserted not upon a ridge but on a rugosity 

 upon the middle of the shaft. It accomplishes chiefly flexion, this 

 being strongly aided by action of the teres major, which with the latis- 

 simus inserts upon a similar but larger and more distal rugosity upon 

 the opposite (medial) side of the shaft. 



In the dugong the humeral head is also posterior to the shaft axis, 

 indicating that flexion and extension is the chief movement, the greater 

 tuberosity is higher than the lesser, the former is located craniad of 

 the head as usual, and both are individually distinct instead of continu- 

 ous as in the manati. In addition there is a heavy and high deltoid 

 crest continuous distad with the greater tuberosity. Humeral condi- 

 tions are essentially similar in the Steller sea cow. 



In the dugong osteological details of the brachium so greatly re- 

 semble those in the sea-lion that I have no choice but render the opinion 

 that the musculature of this region must have many points of similarity. 

 The supraspinous fossa is very much smaller in the dugong, but the 

 muscle should have a comparable action and insert with the infra- 

 spinatus upon the greater tuberosity. Origin 9f the teres minor cannot 

 be as extensive and this muscle must be reduced — perhaps absent. The 

 subscapularis is likely used separately for adduction and there should be 

 a strong deltoid inserting upon the deltoid crest for use in rotation 

 of the humerus. This is all that can be said. In view of the fact that 

 we know so little of the way in which the limb of this animal is mostly 

 used and consequently are unable intelligently to compare its real' 

 function with that of the manati limb, it would hardly be justifiable 

 to advance further possibilities. It may be stated, however, that the 

 anatomical details of the manati indicate that this animal has never 

 used the pectoral limb as a means of propulsion through the water since 

 this member became specialized, while the osteology of the dugong 

 does show that the limb might once have been used either for this func- 

 tion or some other that necessitated muscle action of much the same sort. 



After having discussed the brachium and before taking up details 

 of the antibrachium it is proper to discuss alterations in the length of 

 both of these segments, and consequently in the whole arm, which 

 aquatic mammals have experienced. For the reason that different sorts 

 of vertebrates may respond in such diverse ways to the same stimuli 



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