743 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES 



directly and simply demonstrated on muscle. A long parallel-fibred 

 curarized muscle is supported about its middle; the two ends, which 

 hang down, are connected with levers writing on a revolving drum, and 

 a current is sent longitudinally through the muscle. It is not difficult 

 to see from the tracings that at make the lever attached to the kathodic 

 end moves first, and that the other lever only moves when the contrac- 

 tion started at the kathode has had time to reach it in its progress 

 along the muscle. Similarly, at break the lever connected with the 

 anodic end moves first. The law of polar excitation holds both for 

 striated and for smooth muscle. Not only is there no excitation of 

 unstriped muscle at the anode on closure of the current, but a previ- 

 ously existing contraction disappears. For skeletal muscle the make is 

 stronger than the break contraction. It has not been proved that this 

 is the case for smooth muscle. 



SECTION III. PHYSICAL AND MECHANICAL PHENOMENA OF THE 

 MUSCULAR CONTRACTION. 



When a muscle contracts, its two points of attachment, or, if it 

 be isolated, its two ends, come nearer to each other; and in exact 

 proportion to this shortening is the increase in the average cross- 

 section. The contraction is essentially a change of form, not a 

 change of volume. The most delicate observations fail to detect 

 the smallest alteration in bulk (Ewald). Living fibres kept con- 

 tracted by successive stimuli can be examined under the microscope ; 

 or fibres may be ' fixed ' by reagents like osmic acid, and sometimes 

 a very good opportunity of studying the microscopic changes in 

 contraction is given by a group of fibres in which the ' fixing ' 

 reagent has caught a wave of contraction, and, so to speak, pinned 

 it down. It is then seen that the process of contraction in the fibre 

 is a miniature of that in the anatomical muscle. The individual 

 fibres shorten and thicken, and the sum-total of 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 XV. 



(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 multiplied 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 ft. 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 



