302 Dr. .1. I',. Haycraft. 



rylindrical threads, sometimes inv. .>tc<l by a sarcolemma. and with 

 nuclei within the fibrils, under the Harcolemma or in both of these 

 situations, and we notice that these threads of (issue are striped or un- 

 striped according to the rapidity of their contraction. In Vertebrate 

 skeletal muscle, they contract quickly and are striped, and the sunn- 

 :i|)|ilies to the adductor of the Terelratula which closes its shell so 

 quickly as sometimes to nip its protruding siphon. In many of the 

 Polychsetffi, in many Lamellibranchs, as in Mytilus, and in slowly 

 moving Ascidia, the fibres are devoid of striation. We see then that 

 the striping of muscular tissue cannot be said in any way to associate 

 itself with any particular "build " of cell ; it may be present in both 

 u, spindle and in a cylindrical thread. When a muscular fibre, iti 

 may be spindle-shaped or cylindrical in shape, is called upon in the 

 process of evolution to contract very quickly, then it becomes striped, 

 the cause of which is the segmentation of the previously cylindrical 

 tibrils into varicose threads. The Swallow in its rapid flight han 

 quickly to see, and catch the passing fly, and the fibrils of its ciliary 

 muscle, simple threads of uniform thickness in some ancestral form, 

 now become beaded and cross-striped. 



Striated muscle may, therefore, be defined as " muscular tissues, 

 the ultimate fibrils of which have become varicose, and this in 

 association with the power of quicker and more active move- 

 ment." 



We can now ask ourselves whether it is not possible to explain 

 this correlation between the segmentation of a muscle and its power 

 of contracting more rapidly, and it will, I think, be seen that a very j 

 simple and straightforward explanation can at once be given. TheT 

 whole subject can be resolved into a question of " mass " ; the larger 

 the contractile element is, the longer time will it take to reach its 

 maximal degree of shortening, so that when a fibril segments into a 

 number of much smaller particles, each one contracting and 

 relaxing on its own account, a considerable amount of time will 

 thereby be gained. We have many examples of the influence of 

 bulk, or mass, upon rapidity of contraction in the case of the gross 

 muscles themselves, the larger animals moving relatively slower than 

 the smaller ones, as when the Hare, in spite of its smaller leaps, can 

 nearly keep pace with the Horse, because its leaps are repeated at 

 much shorter intervals. We can now see how, by simple means, a* 

 muscle can, during its evolution, contract more quickly, but the 

 fundamental explanation of the phenomena of contraction is still to 

 be found. Whether or not we may ever be able to express muscular 

 contraction in terms of those phenomena which we see in the inor- 

 ganic world I am not in a position to say, but this we must all be 

 certain of, that this explanation will result rather from a study of tin- 

 contraction phenomena of the lower and simpler types of con- 



