4 



ANIMAL MECHANICS. 



truth ; but since the heart is, of all the muscles of the body, 

 the most essentially involuntary, and yet possesses striped 

 muscles, we must seek for some other explanation of this re- 

 markable difference between muscles ; and indeed it must be 

 granted that, even if the voluntary and involuntary muscles 

 were invariably striped and unstriped, it would still remain 

 as difficult of explanation as before why a voluntary muscle 

 should differ from an involuntary muscle in so singular a 

 manner. 



On a careful examination of the stripes in muscular fibres, 

 it is found that they arise from a tendency of the fibre to split 

 crossways into thin disks, which give an appearance like that 

 of cleavage in rocks to the whole of the fibres. Judging from 

 the analogy of cleavage, I have come to the conclusion that 

 the striped structure in muscles, or tendency to cleave into 

 disks,, is due to their repeated contraction between two fixed 

 or nearly fixed points of origin and insertion. Mr. Sorby has 

 shown, by examination of the microscopical structure of cleaved 

 rocks, and I have shown by calculations founded on the dis- 

 tortion of their fossils, that the planes of cleavage are at right 

 angles to the lines of maximum compression \ Professor Tyn- 

 dall also has proved, by direct experiments on compressed 

 wax, that the formation of cleavage planes perpendicular to 

 lines of pressure is an universal law of nature. Whenever, 

 therefore, we find in muscular fibres a distinct origin and in- 

 sertion, their contraction between these points will produce 

 the pressure necessary for the developement of cleavage at right 

 angles to the length of the fibres ; and, in like manner, when 

 a muscular fibre returns upon itself, as in the sphincter ani and 

 sphincter vagince, its contraction produces the striped structure. 

 In the case of the heart, as is now well established, the mus- 

 cular fibres form closed circuits, coiling round each ventricle 

 and round the whole heart in such a manner, that their con- 

 traction, like that of a twisted India rubber ring, develop es 



