D 



200 DR. PETTIGREW ON THE MECHANISM OF FLIGHT. 



of the opposite side move together to make one step, the three corresponding and oppo- 

 site feet moving together to form the second step. Other and similar combinations are 



met with in the decapods. ' . . . 



The alternating movements of the extremities is interesting as betokening a certain 



degree of flexuosity or twisting either in the trunk or limbs, or partly in the one and 

 partly in the other. In order to understand the twisting which occurs to a greater or 

 less extent in the bodies and extremities (when present) of all vertebrated animals, it is 

 necessary to reduce the bony and muscular systems to their simplest expression. If 

 motion is desired in a dorsal, ventral, or lateral direction only, a dorsal, ventral, or a 

 ht and left lateral set of muscles acting upon straight bones articulated by an ordinary 

 ball-and-socket joint will suffice. In this case the dorsal, ventral, and right and left 

 lateral muscles form muscular cycles*, contraction on the one aspect of the cycle being 

 accompanied by relaxation on the other, the bones and joints forming as it were the 

 diameters of the cycles, and oscillating in a backward, forward, or lateral direction in 

 proportion to the degree and direction of the contraction f. Here the motion is confined 

 to two planes intersecting each other at right angles. When, however, the muscular 

 system becomes more highly differentiated, both as regards the number of the muscles 

 and the variety of the directions pursued by them, the bones and joints also become more 

 complicated. Under these circumstances, the bones in many instances are twisted upon 



The cyclical arrangement of the fibres here referred to is clearly traceable in the hollow viscera of Vertebrates, 

 particularly in the heart, stomach, and bladder.— "On the arrangement of the Muscular Fibres in the Ventricles of 

 the Vertebrate Heart, with Physiological remarks," by the Author, Phil. Trans. 1864 ; "On the arrangement of 

 the Muscular Fibres in the Tunics of the Stomach in Man and other Mammalia," by the Author, Proc. Roy. Soc. 

 186/ ; "On the Muscular arrangements of the Bladder and Prostate, and the manner in which the Ureters and Urethra 

 are closed," by the Author, Phil. Trans. 1867. 



t The bones and joints, it maybe remarked, are not necessary to locomotion. In the Protozoa or cell-animals this 

 is effected by an amorphous contractile mass ; in the worm, leech, and caterpillar by imperfect muscular fibres 

 continuous upon themselves, as in the hollow viscera of Vertebrates — the cyclical arrangement of the text. The muscle 

 becomes interrupted in the Crustaceans by the interposition of an external, and in the Vertebrata by the addition of 

 an internal skeleton. When, therefore, the external and internal skeletons make their appearance, it is to afford the 

 muscular system additional surface and leverage, and to enable it to act with greater precision in a given direction. 

 The skeleton, since it cannot move of itself, is consequently to be regarded as an adjunct or auxiliary of the muscular 

 system. As thejuiuscles are accurately moulded to the bones and to each other, either directly or indirectly (by tendons), 

 and the joints and muscles move in perfect harmony, while the bones are unyielding or rigid, it follows that the 

 osseous system acts as a break or boundary to the muscular one, — and hence the arbitrary division of muscles into ex- 

 tensors and flexors, pronators and supinators, abductors and adductors. Instead, howe^ r, of dividing the muscles 

 into sets, it would be more intelligible and, I believe, more philosophical to regard them (as has been dour in the text) as 

 forming muscular circles or cycles interrupted by processes of bone, whose articular surfaces transmit the motion which is 

 generated on the one side of the circle to the other. If this plan be adopted, the voluntary system of muscles is 



Jib 



This 



> 



view is favoured by analogy and by the fact that the muscular system in the higher Vertebrates is in a state of tonicity 

 i. e. equally balanced or oscillating between two imaginary fixed points, and ready to act, through its extensors and 

 flexors, abductors and adductors, pronators and supinators, with surprising rapidity, the contraction of the extensors 

 on all occasions involving the relaxation of the flexors, and so of the others. The most highly organised animal may 

 in this sense be regarded as a living mass whose parts (hard, soft, and otherwise) are accurately adapted to each 

 other, every part reciprocating with scrupulous exactitude, and rendering it difficult to determine where moti m begins 

 and where it terminates. 



