124 HISTOLOGY 



Between the myofibrils and completely surrounding them is the sar co- 

 plasm, which is a fluid substance containing interstitial granules, fat drop- 

 lets, and glycogen. It differs from the protoplasm of the muscle cells 

 which is found about the nuclei, and which is cut off from the sarcoplasm, 

 according to Baldwin, by the sarcolemma. The granules have been care- 

 fully studied by Bullard (Amer. Journ. Anat., 1912, vol. 14, pp. 1-46) who 

 discusses their staining reactions and probable composition. The sig- 

 nificance of the interstitial granules could not be determined. The fat 

 droplets are regarded as reserved food material, and they vary in abun- 

 dance according to the quantity of fat in the food. Schafer has found no 

 evidence that the isolated sarcoplasm of insect muscles is contractile, but 

 he readily observed the contractility of isolated myofibrils. Moreover the 

 activity of certain muscles in living embryos begins soon after the fibrils 

 are differentiated. 



In the process of contraction, according to Schafer, the hyaline sub- 

 stance of the myofibril passes from the light segment / into the dark seg- 

 ment Q, so that each sarcomere becomes short and broad. He refers to 

 the photograph of the lowest fibril in Fig. 115 as showing that the dark 

 substance is porous (note the end of the fiber toward the right). The 

 sarcolemma bulges between the successive Krause's membranes, which are 

 brought closer together (Fig. 112, A), and the length of each sarcomere is 

 greatly reduced. The dark band Q may become light through the accumu- 

 lation of hyaline substance within it, and the shortened and condensed J 

 may become quite dark, causing a reversal of the original color relations. 

 The sarcoplasm is said to be forced from between the dilated myofibrils in 

 Q, into /. Others consider that contraction is due to a passage of fluid 

 from the sarcoplasm into the myofibrils, and that the beaded form which 

 the myofibrils often present, results from an intake of fluid through the 

 ultra-microscopic membranes which are supposed to surround them. The 

 latter interpretation is defended by Meigs (Zeitschr. f . allg. Physiol., 1908, 

 vol. 8, pp. 81-120), and vigorously attacked by Schafer (Quart. Journ. 

 Exp. Physiol., 1910, vol. 3, pp. 63-74). The older theories of contraction 

 and the numerous papers on the finer structure of striated muscle are 

 admirably reviewed by Heidenhain (Anat. Hefte, Abth. 2, 1899, v l- &> PP- 

 i-iu). 



Adult muscle is composed of such fibers as have been described in the 

 preceding paragraphs. They are arranged in compact bundles, shown in 

 cross section in Fig. 116. Around all the larger muscles there is a connec- 

 tive tissue sheath, or external perimysium, which extends into the muscle in 

 the form of septa, thus subdividing it into bundles or fasciculi. These 

 septa constitute the internal perimysium, and the connective tissue ex- 

 tends from them around the individual muscle fibers, blending with the 

 sarcolemma. In the connective tissue of the diaphragm, elastic fibers are 



