Mode of Contraction of Voluntary Muscular Fibre. 117 



wrong in stating that the dark transverse strise are always formed 

 by the lateral union of the light spaces. 



This appearance of movement cannot be caused by dark spaces 

 of fibrillse lying immediately below the clear spaces of a set of 

 fibres which are superficial to them. As the movement can be 

 seen in a perfectly fresh and undisturbed fibre, it can also be 

 seen on the individual fibrillse, as I have already stated. 



The contraction of voluntary muscle. 



Hales, Prevost and Dumas, from observations made on the ab- 

 dominal muscles of the frog, considered the contraction of mus- 

 cle to be due to zigzag flexures taking place in each fibre. Pre- 

 vost and Dumas imagined it to be an electrical effect of the pass- 

 age of nervous cords across the fibre at the angles of flexure. 



Professor Allen Thomson repeated the experiments of Hales, 

 Prevost and Dumas, and was led from the observations he then 

 made to consider that the zigzag plicse were not produced until 

 the contraction had ceased in the fibres which were the subjects 

 of it ; he observed single fibres continuing in contraction, being 

 simply shortened and not falling into the zigzag flexures. Pro- 

 fessor Owen was also led to doubt the accuracy of the statements 

 of Prevost and Dumas from noticing that during the contraction 

 of unstriated muscles in some Pilarias and in a Vesicularia, a 

 swelling took place in the centre of the fibre which thus became 

 shorter and thicker. 



Dr. A. Parre observed a similar fact in the unstriated muscles 

 of the Polypifera. 



The admirable researches of Mr. Bowman have left us little to 

 wish for with regard to the nature of the contraction ; I refer to 

 his observations published in the ' Philosophical Transactions ' 

 for 1843. All his observations were made on muscular fibres of 

 animals shortly after death. 



I shall briefly allude to some observations made with reference 

 to this subject on the living and uninjured tadpole. 



In April this year (1848), when observing the circulation in 

 the tail of a tadpole after the disappearance of the gills, I was 

 surprised on noticing that the cross-striated muscular fibres were 

 distinctly visible through the external tegument; the contrac- 

 tions after the animal was somewhat exhausted were slow and 

 beautiful, not uniform throughout, as is the case when the tail is 

 observed immediately after the death of the animal and stripped 

 of its integument : the former is the active, living and voluntary, 

 the latter the passive contraction. 



When the contraction was comparatively slow, the approach of 

 the transverse striae could be seen with extreme distinctness ; the 



