394 DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 



The twisting of the wing upon itself during its action, to which I have fre- 

 quently directed attention, is occasioned in the bat and bird by the insertions 

 and direction of the muscles— by the spiral configuration of the articular sur- 

 faces of the bones of the wing, and by the rotation of the bones of the arm, 

 forearm, and hand upon their long axes. In the insect it is due to the insertions 

 and direction of the muscles, and the conformation of the shoulder-joint, this 

 being furnished with a system of check-ligaments, and with horny prominences 

 or stops, set, as nearly as may be, at right angles to each other, and fashioned 

 so as to necessitate the wing acting in the manner specified. 



To confer on the pinion the multiplicity of movement which it requires, it 

 is supplied with a double hinge or compound joint, which enables it to move 

 not only in an upward, downward, forward and backward direction, but also at 

 various intermediate degrees of obliquity. An insect furnished with wings thus 

 hinged may, as far as steadiness of body is concerned, be not inaptly compared 

 to a compass set upon gimbals, the universality of the wing-movements ren- 

 dering any elaborate attempt at balancing quite unnecessary. 



In the bird the head of the humerus is convex and somewhat oval (not round), 

 the long axis of the oval being directed from above downwards, i. e., from the 

 dorsal towards the ventral aspect of the bird. The humerus can, therefore, 

 glide up and down in the facettes occurring on the articular ends of the coracoid 

 and scapular bones with great facility, much in the same way that the head of 

 the radius glides upon the distal end of the humerus. But the humerus has 

 another motion ; it moves like a liinge from before backwards, and vice versa. 

 The axis of the latter movement is almost at right angles to that of the former. 

 As, however, the shoulder-joint is connected by long ligaments to the body, and 

 can be drawn away from it to the extent of one-eighth of an inch or more, it 

 follows that a third and twisting movement can be performed, the twisting admit- 

 ting of rotation to the extent of something like a quarter of a turn. In raising 

 and extending the wing preparatory to the downward stroke two opposite 

 movements are required, viz., one from before backwards, and another from 

 below upwards. As, however, the axes of these movements are at nearly 

 right angles to each other, a spiral or twisting movement is necessary to run 

 the one into the other — to turn the corner, in fact. 



From what has been stated it will be evident that the movements of the 

 wing, particularly at the root, are remarkably free, and very varied. A directing 

 and restraining, as well as a propelling force, is therefore necessary. 



Such complex force is to be found in the voluntary muscles which connect the 

 wing with the body in the insect, and which in the bat and bird, in addition to 

 connecting the wing with the body, extend along the pinion even to its tip. 

 It is also to be found in the musculo-elastic and other ligaments. I do not 

 propose entering upon a consideration of the muscular system of the wing of 



