THE PECTORAL LIMB 



in those sorts examined, but because of the shift of the head this process 

 is so located as to allow the subscapularis, which inserts upon it, to act 

 even more directly as an adductor than usual. If the habitual posture 

 of a humerus be vertical (to the body axis) then the subscapularis can 

 operate efficiently upon a lesser tuberosity that is either low or poorly de- 

 fined. If the humerus be held markedly abducted then equal efficiency 

 will demand a lesser tuberosity of great height and projection. The lat- 

 ter is the case in such an odontocete as Tursiops, in which the tuber- 

 osity is just about as large as, and higher than, the head, indicating in 

 connection with the lateral situation of the latter that the normal posture 

 of the flipper may be a pronouncedly abducted one. But this process 

 is not so high in a number of other toothed whales, and there is even 

 some variation in its precise situation. 



As there is no occasion to extend the arm beyond an angle of 90 de- 

 grees with the body axis there is not only no muscle corresponding in 

 function to a clavoacromiodeltoid, save a weak mastohum.eralis poorly 

 placed for this purpose, but the supraspinatus, normally an extensor of 

 the humerus, has not only suffered enormous reduction but its function 

 has changed. In those sorts dissected its insertion has shifted mediad to 

 the anterior border of the lesser tuberosity and it accordingly acts as a 

 rotator to elevate the anterior border of the humerus. The infraspinatus, 

 also normally inserting upon the greater tuberosity, has shifted its at- 

 tachment distad and slightly laterad and now is inserted chiefly into a 

 fossa, very characteristic of most odontocetes, situated upon the hum- 

 eral shaft, so that it acts not only as an upward rotator of the anterior 

 border but also effectively as an abductor of the arm. Usually in mam- 

 mals the greater tuberosity has developed into an eminence because of 

 the stimulus supplied by the insertions upon it of the supraspinatus, in- 

 fraspinatus, and teres minor, but in the toothed whales the first two mus- 

 cles have shifted elsewhere and the last does not occur as a distinct di- 

 vision, so the greater tuberosity has ceased to exist as a process strictly 

 homologous with the eminence to which this term is applied in most 

 mammals. 



In a number of toothed whales which I have examined there is some 

 variation in the conditions of the proximal humerus as recounted above. 

 Slight eminences, situated in this or that direction from their situation in 

 the dissected specimens bespeak corresponding muscular variation, and 

 there may actually be a prominence which might be mistaken for a 

 greater tuberosity of slight definition, but it is believed that this is only 

 analogous, rather than homologous, as discussed below. 



[221] 



