ii CHANGE OF FORM IN MUSCLE DURING ACTIVITY 67 



of this muscle just beneath the skin-- is moistened with 

 a 1 2 solution of potassium nitrate, the muscle shrinks 



xlotrli/ together ; if the under surface is stimulated in the' 

 same manner there is little or no result, The whole muscle, 

 however, contracts instantaneously if electrically excited by an 

 induction shock. The cause of this striking reaction is, according 

 to Griitzner, that the sartorius of the frog consists of two layers 

 of different muscle-fibres, of which the upper (sluggish) layer con- 

 fnn-ts more slowly than the under (quick) layer, and while only the 

 Jirxf is excited l>y the potassium salt, both, but e*pn-inlly the 7 /'/>/-, 

 react to the electrical stimulus. So, too, many warm-blooded 

 muscles (particularly if thin, c.ij. muscles of belly, diaphragm), 

 when curarised. If dabbed with salt solution they draw slowly 

 together (peristaltic action) ; but if the same place is electrically 

 excited before, or after, with an induction shock, contraction is 

 convulsive and instantaneous. All this evidence goes to show 

 that a muscle, in many cases, has no physiological unity, but is a 

 mixture of at least two functionally different elements, which, in 

 the normal movements of the animal, serve for distinct purposes, 

 as appears from the correspondence of the mode of movement of 

 an organ, or single muscle, with the number of quick or sluggish 

 fibres which characterise its movement. It is also instructive 

 that (as Eollett pointed out) there are, besides the thoracic fibrils 

 characterised by their extremely rapid contraction in the 

 wings of certain insects, other muscles, which are quite distinct 

 anatomically and physiologically, and are of very inconsiderable 

 bulk as compared with the others (the wing-muscles proper). The 

 presence of two kinds of muscles is in obvious relation with two 

 distinct actions. One of these is the unfolding of the wing- 

 apparatus, arrangement of the wing-cases, and spreading of the 

 wings. This action resembles the leg-movements. The second 

 action, on the contrary, is that of actual flight, which has been 

 shown by Marey to depend upon an extraordinary frequency of 

 the beat of the wings in insects. In this case the anatomical 

 difference between quick and sluggish fibres is very significant, 

 and the thoracic fibrils are accordingly distinct on anatomical 

 as well as physiological grounds. They comprise to a certain 

 extent all those properties which are, as a rule, characteristic of 

 muscles that contract permanently and quickly, i.e. great abund- 

 ance of sarcoplasm, and a marked development of cross- striation. 



