240 DR. PETTIGREW ON THE MECHANISM OF FLIGHT. 



a position to give the effective or down stroke, which is delivered downwards and forwards. 

 as in the insect. These movements are reversed during flexion. The action of the bat's 

 wing at the shoulder is particularly free, partly because the shoulder-joint is universal in 

 its nature, and partly because the scapula participates in the movements of this region. 

 The freedom of action referred to enables the bat not only to rotate and twist its wii 

 whole, and so diminish or increase the an°>le which it makes with the horizon, but to rotate 



» 



or screw it in an upward and downward direction, and likewise in a forward and backward 

 one. The rotatory or twisting movement of the wing is an essential feature in flight, as 

 it enables the bat (and this holds true also of the insect and bird) to balance itself with 

 the utmost exactitude and to change its position and centre of gravity with marvellous 

 dexterity. The movements of the shoulder-joint are restrained within certain limits by a 

 system of check-ligaments, and by the coracoid and acromian processes of the scapula ; and 

 the wing is recovered or flexed by the action of an elastic ligament extending between the 



O""'" 1 ^ V^.VXXU^j, 



shoulder and the elbow along the anterior margin of the wing, and by elastic and fibrous 

 structures situated between the fingers and in the substance of the wing generally. The 

 bat flies with great ease and for lengthened periods. Its flight is remarkable for its 

 softness, in which respect it surpasses the owl and the other nocturnal birds. The action 



of the wing of the bat, and the movements of its component bones, are essentiaUy the 

 same as in the bird. 



THE WINGS OF BIEDS. 



Traces of Design in the Wing of the Bird— the arrangement of the primary, secondary, 

 and tertiary feathers, <^.-There are few things in nature more admirably constructed 

 than the wing of the bird, and perhaps none where design can be more readily traced. 

 Its great strength and extreme lightness, the manner in which it closes up or folds during 

 flexion, and opens out or expands during extension, as well as the manner in which the 

 leathers are strung together and overlap each other in divers directions to produce at 

 one time a solid resisting surface, and at another an interrupted and comparatively 

 — resisting one, present a degree of fitness to which the mind must necessarily revert 



ith pi 



If the feathers of the wing only are contemplated, they may be 



upper 



consists 



meat j- dmded into three sets of three eaeh (on hoth sides of the wing,-™ u^ 



dorsal set Plate XV. fig. 63 d, e,f), a lower or ventral set (a, a, 6), and one which _ 



o ; , ^ , dWSi0n ^ int6nded t0 refer the feat «<- to the bones of the arm, 



torearm, and hand, but is more or less arbitrary in its nature. The lower set or tier 

 fih™. f r Pnmary (5) ' SeCOndar y W' ^ tertiary (e), feathers, strung together by 

 furnl 1 m a ^ * hat thCy m ° Te in an onward or inward direction, or 



of feX n v T*' * PreCiSe1y the Same iMtM,t of time,-the middle and upper sets 

 are ^itT;. ^ >, P ° P™" 7 ' 8econdM y' and t««ary ones, constituting what 



loX" L f t \ "f " SubcOTerts '' ^c primary or rowing-feathers are the 



erti r es ho " ^ Se °° ndarieS <°> *»'» and the tertiaries third (a). The 



S^ni T7k are ° CCaS10IlalI y lon S« than the secondaries. The tertiary, secondary, 



towards th J - ' TT " StreDgth fr0m With - Awards, i. e. from the body 



towards the extrcmuy of the wing, and so of the several sets of wW-eoverts. This 



