STRIATED OR VOLUNTARY MUSCLE. 



461 



represented by the intermediate and median bands. The foregoing diagram ( Fig. 

 476), modified from Heidenhain, indicates the relations of the several bands to be 

 seen in muscle when examined under the most favorable conditions. That various 

 reagents produce marked changes in the details of the muscle-picture admits of no 

 question ; this has been graphically represented by the last-quoted author.^ The fact 

 that the intermediate disk is attached to the sarcolemma is shown by the constrictions 

 or scalloped margin in the outline of the fibre during contraction, the constrictions 

 corresponding in position to the attachment of the membranes of Krause. The 

 striped muscle of certain insects exhibits an additional band, the accessory disk, sub- 

 dividing the light zone { J ). 



The distribution of the contractile fibrillae throughout the fibre is not uniform, 

 since the fibrillae are grouped into bundles, the viuscle-coliunns or sarcostyles. This 

 arrangement is well shown in suitably prepared transverse sections of muscular tissue 

 (Fig. 479), in which the individual fibres are seen to be made up of minute stippled 

 areas separated by clear lines. These areas are known as Cohnheh7{ s fields, and 

 represent the transversely cut groups of contractile fibrillae. The clear lines indicate 

 the distribution of the sarcoplasm ; in addition to forming the net-work dividing 

 Cohnheim's fields, the sarcoplasm separates the groups of individual fibrillae, each 

 muscle-column being entirely surrounded by the less highly differentiated substance. 



Fig. 478. 



1 Sarcolemma 



Cohnheim's 

 field 



Muscle- 

 nuclei 



Endomysium 



Nuclei of interfibr 

 tissue 



Diag:ram illustrating Rollet's view of structure of 

 muscle-fibre and relations of assumed details to usual 

 appearances of tissue. 



Muscle-fibres of lizard in transverse section, showing 

 fields of Cohnheim and muscle-nuclei. X 650. 



When seen in longitudinal section, the sarcoplasm between the groups of fibrillae 

 appears as lines extending the entire length of the fibre, to which an inconspicuous 

 longitudinal striation is thus imparted. 



The muscle-fibre has already been spoken of as a multinucleated cell. The 

 nuclei resulting from the division of the nucleus of the embryonal cell remain within 

 the sarcoplasm and are termed muscle-nuclei. Their position in mammalian muscle 

 is usually immediately beneath the sarcolemma ; in certain fibres, however, as those 

 composing the semitendinosus of the rabbit (Fig. 480), and of uncertain distribution 

 in man, the nuclei lie more deeply embedded within the sarcoplasm, therein agreeing 

 in location with the position occupied by the nuclei in the muscular tissue of many 

 of the lower vertebrates (Fig. 479). 



Variations in the color and contractility of muscular tissue have been described 

 by Ranvier and Krause, Klein, Griitzner, and others. While the skeletal muscles 

 are usually of a pale tint and contract energetically when stimulated, particular mus- 

 cles of certain animals, as the semitendinosus and the soleus in the rabbit, possess a 

 deeper color and contract more slowly and prolongedly under stimulation. Such 

 red muscles, as they have been named, are composed of fibres which are thinner than 

 common and possess a relatively larger amount of sarcoplasm, in which the muscle- 

 nuclei are embedded not only immediately beneath the sarcolemma, but also in the 



^ M. Heidenhain : Anatom. Anzeiger, Bd. xx., Nos. 2 and 3, 1901. 



