PHYSIOLOGY. 63 



CHAP. II. OF THE ACTION OF MOVING FIBRES. 



LXXXI. The moving fibres (XXIX. 4), so far as yet 



known, are of one kind only, and the same every where, as in 

 the most commonly known muscles. Hence, the terms moving 

 and muscular fibres are of the same import. 



" What a muscle is, is well known from anatomy. I suppose, 

 also, that it will be very well known, that these muscles are 

 composed of bundles of separate and distinct fibres, that the 

 separation goes to a great degree of minuteness, and that the 

 motion which occurs in the whole muscle, depends upon the 

 motion of those fibres of which it consists. What we know 

 with respect to muscular fibres in general, has been learned 

 from observation and experiment upon these muscles ; but it 

 has been supposed that they are different in various animals, 

 and in different parts of the body in the same animal ; and I 

 allow that this may perhaps be so, but so far as our observa- 

 tions, strongly supported by analogy, yet go, the presumption is, 

 that they are every where of the same nature ; and anatomists are 

 united in this, that although in many cases we cannot distinctly 

 discover the fibres, or their appearance, we may conclude as 

 to the structure of the whole wherever there is irritability. I 

 go on to use the term muscular fibre, because I suppose that it 

 is the moving fibre every where, and that it is every where of 

 the same nature ; we can trace it to the alimentary canal, to the 

 fibres of the bronchise in the lungs, and over the whole vascular 

 system ; and where we cannot trace it, we presume upon its 

 being present from the properties, of which we find the several 

 parts possessed." 



LXXXI I. A muscular fibre is supposed to have a peculiar 

 organization, different both from that of the simple solid fibre, 

 and from that of the medullary fibres in every other part of the 

 nervous system ; but wherein that peculiarity of organization 

 consists is not yet ascertained. 



LXXXIII. A muscular fibre is endued with a contractility 

 which is different from that of the simple solids, or of any inan- 

 imate elastics ; especially in this, that the contraction of a mus- 

 cular fibre is excited by causes which do not affect these others. 



