638 A MANUAL OF PHYSIOLOGY 



caught a wave of contraction, and, so to speak, pinned it down. 

 It is then seen that the process of contraction jin the fibre is a 

 miniature of that in the anatomical muscle. I The individual 

 fibres shorten andjjthicken, and the sum-total of c this shortening 

 and thickening is the muscular contraction which we see with the 

 naked eye. The phenomena of the muscular contraction may 

 be classified thus : (i) Optical, (2) Mechanical, (3) Thermal, 

 (4) Chemical, (5) Sonorous, (6) Electrical. (5) will be treated 

 under ' Voluntary Contraction ' ; (6) in Chapter XI. 



(i) Optical Phenomena Microscopic Structure of Striped Muscle. 

 The structure of striped muscle has long been the enigma of histology ; 

 and the labours of many distinguished men have not sufficed to 

 make it clear. On the contrary, as investigations have multiplied, 

 new theories, new interpretations of what is to be seen, have multi- 

 plied in proportion, and a resolute brevity has become the chief duty 

 of a writer on elementary physiology in regard to the whole question. 



The muscle-fibre, the unit out of which the anatomical muscle is 

 built up, is surrounded by a structureless membrane, the sarcolemma. 

 The length and breadth of a fibre vary greatly in different situations. 

 The maximum length is about 4 cm. ; the breadth may be as much 

 as 70 ^ and as little as 10 /*. When we come to analyze the muscle- 

 fibre and to determine out of what units it is built up, the difficulty 

 begins. The fibre shows alternate dim and clear transverse stripes, 

 and can actually be split up into discs by certain reagents. It also 

 shows a longitudinal striation, and can be separated into fibrils. 

 Some have supposed that the discs are the real structural units 

 which, piled end to end, make up the fibre. The fibrils they con- 

 sider artificial. This view is erroneous. It seems certain that the 

 fibres are built up from fibrils ranged side by side, and that the discs 

 are artificial. The contents of the muscle-fibre appear to consist of 

 two functionally different substances, a contractile substance, and an 

 interstitial, perhaps nutritive, non-contractile material of more fluid 

 nature. The contractile substance is arranged as longitudinal fibrils 

 embedded in interfibrillar matter (sarcoplasm) . In a muscle im- 

 pregnated with chloride of gold the interfibrillar matter appears as a 

 network. 



Schafer has described the contractile elements of the muscle-fibre 

 (Figs. 224, 225) as fine columns (sarcostyles), divided into segments 

 (sarcomeres) by thin transverse discs (Krause's membranes), occupy- 

 ing the position of the middle of each light stripe. Each sarcomere 

 contains a sarcous element (a portion of the dark stripe) with a clear 

 substance at its ends, filling up the space between the sarcous 

 element and Krause's membrane, and constituting a portion of the 

 light stripe. The sarcous element is itself double, and if the fibre 

 be stretched, the two portions separate at a line which runs trans- 

 versely across the middle of the dim stripe (Hensen's line). Schafer 

 considers that the appearance of longitudinal fibrillation in the sarcous 

 elements is due to the presence in them of fine longitudinal canals or 

 pores. 



Rutherford has given a somewhat different account of the matter. 

 According to him, each fibril is made up of a longitudinal row of 

 segments of two kinds, alternating with each other (Fig. 226) : 

 (i) ' Bowman's elements,' shaped like an elongated hour-glass, and 



