122 



HISTOLOGY 



part, or true sarcolemma, is a structureless membrane closely applied to 

 the surrounding connective tissue. It appears to be much more definite 

 than any membrane which invests smooth muscle fibers, to which the term 

 sarcolemma has been extended by Heidenhain and others. The sarco- 

 lemma of striated muscle, however, is not yet thoroughly understood. 

 Although the muscle cells are generally said to be within it, Baldwin finds 

 that they are outside of the sarcolemma, between it and the fibrous base- 

 ment membrane (Fig. 112, A). Accordingly he agrees with Apathy in 

 regarding the myofibrils as comparable with connective tissue fibers. The 

 possibility that the myofibrils are intercellular will be discussed under 

 cardiac muscle. 



The appearances of skeletal muscle which have caused it to be called 

 striated are found only in longitudinal sections, including those which are 

 obliquely longitudinal. It is then seen that the myofibrils, which run 

 lengthwise, are composed of alternating light and dark portions, and that 

 they are so arranged that the dark parts of one fibril are beside the dark 

 parts of the adjacent fibrils. As a result of the close crowding of the fibrils, 

 alternating light and dark transverse bands appear to pass from one side 

 of the fiber to the other, and these are the striations. They are shown in 

 Fig. 1 1 2, A and B (at the right of A, the fibrils are represented as artificially 

 frayed apart). 



Bowman (1840) stated that "a decisive characteristic of voluntary muscle consists 

 in the existence of alternate light and dark lines, taking a direction across the fasciculi." 



He added that Leeuwenhoek had described 

 the striae repeatedly, believing in the earlier 

 years of his inquiry that they were circular 

 bands around the fibrils, but later regarding 

 them as of spiral arrangement, comparable 

 with an elastic coil of wire, and in some way 

 capable of retraction. Bowman recognized 

 that they were caused by the " coaptation of 

 the markings of neighboring fibrillae." He 

 found that the muscle fibers can readily be split 

 into longitudinal fibrillae with transverse mark- 

 ings, but that "in other cases their natural 



cleavage is into discs, and in all instances these discs exist quite as unequivocally as 

 the fibrillae themselves." The discs are produced when the ends of a muscle fiber 

 are pulled apart (Fig. 113). Bowman regarded each disc as a plate of agglutinated 

 segments, receiving a single segment from every fibrilla which crossed it. These seg- 

 ments he named sarcous elements; they are united endwise to form the myofibrils and 

 crosswise to form the discs. Usually the longitudinal cohesion is much greater than 

 the lateral, and in the wing muscles of insects, according to Schafer, the fiber "never, 

 under any circumstances, cleaves across into discs." 



The finer structure of the fibrils is illustrated in the diagram, Fig. 114, 

 which represents a part of seven myofibrils, including three dark bands 



FIG, 113. A HUMAN STRIATED MUSCLB FI- 

 BER SEPARATING INTO Discs. (After 

 Bowman.) 



