324 DP PETTIGREW ON THE PHYSIOLOGY OF WINGS. 



catecl by Professor Huxley to the Linnean Society, and read before that body 

 on the 6th and 20th of June 1867. It is published in extenso in the 26th 

 volume of the Transactions of the Society, with upwards of eighty illustrations/"' 

 The principal object of the memoir is to establish an analogy between the 

 walking surfaces of quadrupeds, the swimming surfaces of fishes, and the flying 

 surfaces of insects, bats, and birds. These are all described! and figured \ as 

 twisted levers or screws in an anatomical sense (pages 361 and 362, figures 37, 

 38, 39, and 40), and as flexible reversing screws in a functional or physiological 

 sense (pages 336 and 362, figures 2, 41, 42, and 43). § As a consequence, 

 the quadruped and biped || are represented as walking, IT and the seal and 



* On the Mechanical Appliances by which Flight is attained in the Animal Kingdom, &c. 



t Op. eit., from page 199 to page 267 inclusive. 



t Op. cit., Plate XV. figs. 49, 51, 57, 68, 69, 70. Likewise Diagram 18 A d'e'f, a'b', page 253. 



§ Op. cit, Plate XV. figs. 58, 59, 61, 73, 74, and 75. 



|| Op. eit., Plate XV. fig. 78. 



U I think it proper to state that various anatomists have carefully examined the form of the articular 

 surfaces of the joints hi the limbs, more especially in man. The researches of the brothers "Weber and 

 Professor Meyer of Zurich are so well known, that it may suffice simply to refer to them. I would also 

 direct attention to the writings of Langer, Henke, Meissner, and the late Professor Goodsir, Langer, 

 Henke, and Meissner succeeded in demonstrating the " screw configuration" of the articular surfaces of the 

 elbow, ankle, and calcaneo-astragaloid joints, and Goodsir showed that the articular surfaces of the knee- 

 joint consist of " a double conical screw combination." The last-named observer also expressed his belief, 

 " that articular combinations, with opposite windings on opposite sides of the body, similar to those in 

 the knee-joint, exist in the ankle and tarsal, and in the elbow and carpal joints ; and that the hip and 

 shoulder joints consist of single-threaded couples, but also with opposite windings on opposite sides of 

 the body." The following are the views of Langer as interpreted by Goodsir : — (Proc. Eoy. Soc. Edin., 

 Jan. 18, 1858, and Anatomical Memoirs, vol. ii. p. 231.) " Langer, acting on the happy idea of pro- 

 longing the screw by uniting, in one direction, a number of plaster casts of the same articular surface, 

 succeeded in forming continued screws from the upper articular surface of the astragalus in the horse, 

 panther, and human subject. Langer concludes that the ' go line ' (a line obtained from the scratch of 

 a steel point fixed on one of the articular surfaces, and which marks the opposite surface when the joint 

 is moved) of the ankle-joint in all the mammalia is a portion of a helix, and that therefore the astraga- 

 loid surface is a segment of a cylindrical or conical male screw, while the tibio-fibular surface is a 

 segment of the corresponding female screw. The right ankle-joint is a left-handed screw combination ; 

 the left ankle-joint a right-handed. "When therefore the foot is conceived to be fixed, the leg, in passing 

 from a position of extension to flexion, moves laterally outwards along the axis of rotation, and the sine 

 of the angle of inclination of the thread — that is, in proportion to the extent of flexion and the rapidity 

 of the screw." Goodsir, in attempting by Langer's method to develop those articular screw-models, found 

 that when two casts were united, an apparently satisfactory helix was produced ; but in adding to the 

 series, the spire diminished, and the helix closed upon itself ; so that it appeared that not only the 

 angle of inclination of the thread, but also the radius of rotation, diminished. He was, therefore, of 

 opinion, that the tibio-astragaloid articular surfaces could not be regarded as segments of a cylindrical 

 series, and thought it extremely probable that, abstracting the terminal facets, the acting areas on each 

 surface consist each of a segment of a conical screw — the convex portions of these two screws being on 

 the astragaloid, the concave on the tibial articular surface ; the one screw coming into action in flexion, 

 the other in extension. Goodsir's experiments on the knee and ankle-joints, conducted with extreme 

 care, by the aid of fresh specimens, casts, and models, led him to conclude that both joints were ' spiral 

 in their nature' — that hi fact they were ' screwed structures,' and that the movements of the knee-joint 

 are combined gliding and rolling movements of conical screwed surfaces upon one another. The follow- 

 ing are his own words : — " The general character of the curves observed, and the corresponding move- 

 ments and structure of the joint (knee-joint) leave little doubt in my mind that the flexion and extension, 

 combined gliding and rolling movements of the knee, are performed between two conical double-threaded 

 screw-combinations, an anterior and a posterior — the anterior being a left-handed screw, and the posterior 



