i ORGANISATION AND STRUCTURE OF MUSCLE 23 



direction are connected by fine protoplasmic bridges, between 

 which are small intercellular spaces (Fig. 14). 



Common as is a marked pigmentation in the uninuclear, 

 invertebrate muscles, it is but seldom that we find definite colora- 

 tion of the smooth muscle-cells of vertebrates. The bright red 

 muscles of the gizzard of many birds are, however, an exception, as 

 well as the contractile fibre-cells, crowded with dark-brown pigment 

 granules, in the iris of many fishes and amphibia, which, according 

 to Steinach (15), owe to these their direct excitability to light. 



We must not omit to mention certain very peculiar figures 

 obtained from smooth muscle-fibres that have been fixed during 

 contraction. A highly regular transverse striation is often visible, 

 due presumably to swelling from the contractions that follow 

 one another at regular intervals. The figures were first pointed 

 out by Heidenhain in 1861 (16), and 

 Drasche has recently supplemented our 

 imperfect knowledge of them (17). He 

 observed a regular cross -striation in the 

 individual fibre -cells of the contracted 

 muscular coat of the poison - gland of 

 Salamander, resulting from a delicate trans- FIG. 14. Transverse section 



verse grooving, or marked involution, pro- f . th , e w^er-muscies of 



Rat (cell-bridges). (From 



duced by the contraction of the lower c. <\eBn\yne,Arch.deBwi., 



f f> , i n- -i IT van Beneden, vol. xii. 1892.) 



suriace 01 the muscle, bimilar very delicate 

 figures were observed in the muscle-cells of a cat's intestine 

 that had been hardened in alcohol (Biedermann). The sharply 

 defined, highly refractive, transverse swellings gave an im- 

 pression of local (fixed) waves of contraction, such as occur 

 occasionally in certain striped muscles. 



Apart from the Arthropoda, in which uninuclear muscle- 

 cells seem to be altogether wanting, the existence of true cross- 

 striation, i.e. disposition of each single fibril in layers of different 

 optical relations, would appear from the foregoing to be compara- 

 tively rare in muscle-cells ; its physiological significance has 

 already been indicated. 



The much commoner oblique striation stands, as we have said, 

 in no sort of relation to the transverse striae, since the single fibrils 

 are not further differentiated, and deviate in direction only from 

 the normal. We find it impossible to subscribe to the theory 

 recently advanced by Knoll to the effect that there is no sharp 



