PHYSIOLOGY. 71 



and this probably arises from a stagnation of the fluids in 

 the remaining arteries and beginning veins, which gives a rigi- 

 dity to the parts. I say, therefore, the influx of blood is neces*- 

 sary as a condition to give the flexibility, tension, and warmth ; 

 but the contraction subsists in many animals, after all motion 

 of the blood has ceased. The thorax of a frog being opened, 

 and the great vessels cut through, although nothing like the 

 motion of the blood is subsisting, yet the animal will hop about 

 with alacrity and vigour for hours afterwards. If in the animals 

 of warm blood this experiment does not succeed as readily, it 

 amounts to this, that the power of contraction is more perma- 

 nent in colder blooded animals." 



XCIX. The contraction of muscular fibres does not depend 

 on the inflation of vesicles, or other such analogous structure, 

 as the shortening of the fibres in contraction is often greater 

 than can take place in such structure. "Leuwenhoek started this 

 hypothesis of the vesicular structure, and the ingenious Hook has 

 prosecuted it. They assumed that the fibre was like a string, 

 with such vesicles, and that these vesicles had an elasticity by 

 which they could be drawn out into their several figures, &c. 

 This view readily explained the contraction by the influx of a 

 fluid ; for, suppose that the muscular fibre, and its several vesi- 

 cles, are drawn out to their greatest extent, if you fill up these 

 a contraction will be produced; and almost every hypothesis has 

 supposed this, that the fibres are a portion of artery, and that 

 the influx of blood dilates the cylindrical vessels, and brings 

 them into a spherical figure. But no such structure will ac- 

 count for the contraction which really takes place. Thus, if 

 you fill a cylinder in such a manner that it shall change into a 

 spherical form, which is the greatest contraction possible, it 

 cannot be diminished above one-third of its former length ; but 

 many observations shew that our muscular fibres are contracted 

 to a greater degree ; as in the polypus, which can be contracted 

 to one-twelfth part. We must, therefore, seek for some other 

 structure or organization of muscular fibres. If you will look 

 over the various hypotheses, you will find how many are cut off' 

 by the observations in these two paragraphs, with regard to 

 which I am sufficiently confident. But I am by no means .so 



