Spiral Structure of Muscle. 83 



the fibrils. " Transverse cleavage " of the primitive fasciculus 

 is caused by the spirals giving way at that part where they offer 

 the least resistance, viz. at the part where they cross one another 

 aud are in contact. The sarcolenima arises from the coalescence 

 of spirals. 



It was by attending to the history of development of muscle, 

 chiefly in the larva of the large toad of Jersey, that the author 

 arrived at a knowledge of the foregoing facts. His observations 

 published in the present paper were made with one of the large 

 compoimd microscopes of Plossl ; and it happened that the in- 

 strument was the very last constructed by that justly renowned 

 optician. The following are some of the new facts herewith 

 observed ; others will be mentioned fm'ther on. 



The two spirals of which the muscular fibril consists, run in 

 the same direction, and not in opposite directions, as the author 

 at fii'st supposed. (Plate I. fig. 1.) 



These two spirals are united at the end of the fibril, as in a 

 loop. Such at least is the case in one form of muscle, PI. II. fig. 30, 

 and from analogy it may be presumed to be the same in others. 



The fibril, being thus a double or twin spiral, undergoes a 

 stronger twisting in contraction, and an untwisting in relaxation. 



\^Tien met with in relaxation, the two spirals usually present 

 themselves in a state comparable to that of common twine. 

 (PI. I. fig. 2.) , . . ■ ■ 



Between the untwisting in relaxation and the twistmg m con- 

 traction, there are several intermediate states. Fig. 3 presents four 

 such intermediate states seen in different parts of the same fibril. 



Cilia also are double spiral threads, and thus have a structure 

 like that of the muscular fibril. (Pigs. 29, 30, 31, &c.) 



The author then describes models of lead wire which he has 

 constructed for the purpose of illustrating the structure of the 

 muscular fibril, fig. 1. These models, though very rude ones, 

 may afford some idea of the different conditions of the fibril in 

 regard to length, breadth, and thickness, in different degrees of 

 contraction, a, and relaxation, b. [b, in the model, fig. 1, repre- 

 sents part of a, after the drawing of the latter out*.) 



It is an error to sujjpose it possible to learn how the strise in 

 muscle are produced, by examining merely the ])rimitive fasci- 

 culus. The ])rimitive fasciculus must be, as far as possible, teased 

 out with needles, and the fibrillaj separately examined. To obtain 

 the fibril, muscle should be selected in which the primitive fas- 

 ciculi are small. For this purpose the heart is especially to be 



♦ [Such models have since been presented to the Royal Society and to 

 the lloyai College of Surgeons in London by the Author. — May 18.52.] 



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