460" 



HUMAN ANATOMY. 



or anisotropic, substance is succeeded at either end by the Hght band {J J) of singly- 

 refracting, or isotropic, substance. The hght band is subdivided by a delicate line, 

 the intermediate disk (Z ), also known as Krause s membrane. The sequence which 

 by repetition makes up the contractile fibrilla consists, therefore, of Z +/+ Q -iJ^Z. 

 Under favorable conditions for examination the transverse disk exhibits less density 

 midway between its ends ; this zone is traversed by a delicate line (M), the media^i 

 disk (Hensen, Merkel) or middle membrane (M. Heidenhain). 



The interpretation of these appearances, shown as usually seen under moderate 

 amplification in the accompanying photograph (Fig. 477), has been the subject of 

 much laborious investigation and vexed discussion ; even at the present time authorities 

 are far from accord as to the significance of the observed details in their relations to the 

 architecture of the muscle-fibre. It is beyond the purpose of these pages to review 

 the various theories concerning the ultimate structure of striped muscle ; ^ suffice it to 

 point out that, apart from the conclusions of those observers who from time to time 

 have contended that the appearances are entirely optical and do not correspond to 



actual structural details, two chief 



Fig. 477. 





0^ 



views regarding the architecture of 

 the muscle-fibre have been held. 

 According to the one, championed 

 by Krause, the intermediate zone 

 is regarded as the expression of 

 a membranous septum which 

 stretches entirely across the mus- 

 cle-fibre as an inward extension of 

 the sarcolemma and thus subdi- 

 vides the fibre into a number of 

 minute compartments, or contrac- 

 tile disks, by the longitudinal ap- 

 position of which the entire fibre 

 is built up. The other view, early 

 accepted by KoUiker, regards the 

 fibre as made up of fibrillcB ex- 

 tending the length of the fibre, 

 the transverse cleavage into disks 

 being secondary and artificial. The 

 fibrillar theory as advanced by Rol- 

 let has received wide acceptance 

 and deserves brief mention. Ac- 

 cording to this authority, the con- 

 tractile fibrillae are to be conceived 

 as forming anisotropic rods consisting of alternating thicker and thinner segments 

 (Fig. 478), the former corresponding in position with the broad, dark, transverse 

 disk, the latter with the lighter band, since the meagre amount of doubly refracting 

 substance in this zone is masked by the large quantity of isotropic sarcoplasm. 

 Rollet recognized the mtermediate disk as consisting, not of a continuous membrane, 

 but as an interrupted line representing a row of minute beads which exist as local 

 accumulations on the thinner segments of the fibrillse. Rollet' s conception of the 

 fibre, therefore, included the sarcolemma containing the sarcoplasm in which the con- 

 tractile fibrillae were embedded. 



More recent investigations with the aid of improved differential stains have led 

 to a modification of the fibrillar view in so far that the intermediate disk is to be 

 regarded as a structure that is attached to the sarcolemma and extends between the 

 fibrilke. M. Heidenhain believes the median disk to be an additional membrane that 

 likewise meets the sarcolemma at the periphery of the fibre. The later conception 

 of muscle architecture in no wise questions the existence of the fibrillae as the con- 

 tractile elements of the fibre, but regards them as held in place by the lateral braces 



^ An exhaustive review of the literature and various opinions regarding the structure of 

 striped muscle is given ty M. Heidenhain: Ergebnisse der Anatomic und Entwick., Bd. ix.. 

 1899. 



Photograph of striated muscle, showing the usual appearance 

 under moderately high magnification. < 700. 



i 



