300 MANUAL OF HISTOLOGY. 







The kind of contraction, again, varies according to the histological 

 elements with which we are engaged. In striped fibres we observed it to 

 commence almost simultaneously with the application of the excitant to 

 the nerves of the part, ceasing very rapidly again on cessation of the 

 stimulus, and giving way to relaxation. The reverse may be observed in 

 smooth muscle. Here an appreciable interval of time is remarked be- 

 tween the application of the stimulus and contraction, while the latter 

 outlasts the action of the excitant for some moments; the fibre reassum- 

 ing gradually a condition of quiescence. This is evident in the motions 

 of whole groups of animals, and also in those of individual organs, as in 

 the iris of birds, made up of striped fibres in contrast to that of human 

 beings and mammals generally, in which the former are smooth. With 

 us it is the striated fibres alone which obey the influence of the will 

 in their rapid and precise action. 



In the rectilinear muscle, with the aid of the microscope, we see during 

 contraction the longitudinal stria? become less distinct, and eventually 

 disappearing, while the transverse markings become clearer and clearer. 

 It would naturally be a great point achieved could we ascertain precisely 

 how the elementary particles of the active fibre are affected by this, and 

 especially how the dark zones behave in relation to the clear. It appears, 

 however, as though the former approached each other, while the clear zones 

 decreased in height. These points are, however, still too doubtful for us 

 to be able to draw any great conclusions from them. We regard it as not 

 improbable, however, that the sarcous elements may be relatively immut- 

 able, as compared with the particularly contractile longitudinal cement- 

 ing medium. According to Amicfs observations on the muscles of the 

 common fly, the elongated fleshy particles appear to assume an oblique 

 position at the moment of contraction. This we have ourselves seen. 



According to the most recent observer, W. Engelmann, the seat of the 

 contracting force is exclusively the dark (anisotropic) layer. The trans- 

 parent (isotropic) transverse zone is either contractile in a minor degree, or 

 only elastic probably, like the dark transverse disk of Krause. While 

 the volume of the muscle casket enclosed between two of the latter does 

 not become appreciably lessened, the dark transverse zone at the moment 

 of contraction increases in bulk, the clear becomes less voluminous ; the 

 first swells, the last shrinks, so that an overflow of fluid takes place. 

 Besides this the first becomes clearer and softer, the latter darker and 

 more solid. 



The sarcolemma, owing to its elasticity, follows the changes of form in 

 the fibre, tightly investing the latter throughout. That the transverse 

 striai are not produced by wrinkles across its substance was recognised 

 long ago. The motor nerves will be referred to in a future chapter. 



It is a matter of far greater difficulty to obtain a view of the contractile 

 fibre-cell, or unstriped fibre, in the moment of contraction. According 

 to Heidenhain, each element (at least among invertebrate animals) be- 

 comes likewise simultaneously and evenly thicker in all its parts, with a 

 corresponding decrease in length. 



As to the rigor mortis connected with the death of muscle, on which 

 an albuminous substance contained in the latter undergoes coagulation, 

 while its reaction becomes acid, the microscope has added but little to 

 our knowledge. The dead fibre appears more rigid and dull, and less 

 transparent than during life. 



