612 



VERTBBRATBD ANIMALS. 



of which is composed of a single row of the primitive particle 

 (Fig. 325). The diameter of the fibres varies greatly in different 



kinds of Vertebrated ani- 

 FiG. 325. mals. Its average is greater 



in Reptiles and Fishes than 

 in Birds and Mammals, and 

 its extremes also are wider; 

 thus its dimensions vary in 

 the Frog from 1-lOOth to 

 1-lOOOth of an inch, and in 

 the Skate from l-65th to 

 l-300th; whilst in the Hu- 

 man subject, the average is 

 about l-400th of an inch, 

 and the extremes about l-200th and l-600th. 



426. When the fibrillse are separately examined, under a 

 magnifying power of from 250 to 400 diameters, they are seen 

 to present a ojdindrical or slightly beaded form ; 

 and their linearly aggregated pai'ticles then appear 

 to be minute cells. We observe the same alterna- 



Slrialed Muscular filire, separating into fibrillEe. 



Fig. 326. 



a 



ea 



tion of light and dark spaces, as when the fibrillse 

 are united into fibres or into small bundles ; but 

 it may be distinctly seen, that each light space is 

 divided by a transverse line ; and that there is a 

 pellucid border at the sides of the dark spaces, as 

 well as between their contiguous extremities (Fig. 

 326). This pellucid border seems to l)e the cell- 

 wall; the dark space enclosed by it (which is 

 usually bright in the centre) being the cavity of 

 the cell, which is filled with a highly refracting 

 substance. When the fibril is in a state of relaxa- 

 tion, as seen at a, the diameter of the cells is 

 greatest in the longitudinal direction: but when 

 it is contracted, the fibril increases in diameter as 

 it diminishes in length; so that the transverse 

 diameter of each cell becomes equal to the longitu- 

 dinal diameter, as seen at h ; or even exceeds it. 

 Thus the act of ]\Iuscular contraction seems to 

 consist in a change of form in the cells of the 

 ultimate fibrillse, consequent upon a contraction 

 between the walls of their two extremities; and it 

 is interesting to observe, how very closely it thus 

 corresponds with the contraction of certain Vege- 

 table tissues, of which the component cells are 

 capable of producing movements, when they are irritated, by' 

 means of a similar change of form. The diameter of the ulti- 

 mate fibrillffi will of course be subject to variations, in accord- 

 ance with their contracted or relaxed condition ; but it seems to 

 be otherwise tolerably uniform in different animals, being for 



Slruclure of the 

 ultiiriale Fibrill<B 

 of Strutted Muscu- 

 lar Jibre : — a, a 

 fibril in a state of 

 ordinary relaxa- 

 tion ; ft, a fibril in 

 a slate of partial 

 contraction. 



