THE PHYSIOLOGY OF MUSCLE 487 



galvanic shocks and record on the drum going at the minimal speed of 

 its mechanism. Compare (1) its latent period (several seconds) with 

 that of striated muscle; (2) its period of contraction (thirty seconds or so); 

 (3) its period of relaxation (one minute or so); (4) the general shape of 

 the curve. (All these quantities are very variable with many varying 

 conditions.) These two sorts of muscle are obviously very different in 

 their functional habits, yet they are less different than is sometimes sup- 

 posed. (The "intermediate" variety, that of the heart, will be studied 

 later on.) 



To understand the great differences between the modes of contraction 

 of cross-striated and of smooth muscle, it is necessary to recollect their 

 respective structures. A smooth muscle-fiber is functionally all one 

 piece of protoplasm, massive, and with relatively much inertia. A 

 cross-striated muscular fiber, on the other hand, is made up of very 

 numerous minute portions, each of which, so far as function (contraction), 

 is concerned, is practically a separate muscle-fiber, and one constructed 

 with the greatest readiness for action. Thus, in cross-striated muscle 

 the inertia is divided into many parts, and the muscle therefore contracts 

 very rapidly and very vigorously. The difference between them is 

 not unlike that between a galvanic battery made up of numerous small 

 cells and one composed of only one cell wjth^ 



trical organ of electrical fishes makes this comparison apt, for it is cross- 

 striated muscle modified to produce an electric shock of high intensity, 

 and composed of hundreds of cells like the former of the two batteries 

 above suggested. In a striated muscle the effect is usually sudden and 

 powerful and of relatively short duration, as its structure implies, while 

 in smooth muscle, acting with large and relatively few elements (cells) 

 the contraction is generally the relatively slow, steady, contraction and 

 relaxation we should expect to find in the vegetative, as distinguished 

 from the voluntary organs. In this particular experiment with the 

 frog's stomach the difference between the two sorts of muscle is perhaps 

 unfairly exaggerated because these smooth fibers are circular fibers, and 

 thus give only half as much linear shortening when the rings are thus 

 connected as they would if spread out their full length as are the fibers 

 in the cross-striated muscle compared with them. 



The danger in this experiment is that the contraction will not be 

 appreciated because of its small degree and its very long periods. 

 Sometimes an active relaxation apparently complicates matters. More- 

 over, the stomachs of winter frogs may be very loth to react, either way. 



Expt. 44. Sudden Change and Gradual Change of the Strength of 

 Stimulus. (Apparatus: Myograph, rheocord, two simple keys.) (A) 

 Sudden Change. Set up the apparatus carefully as follows: Put two 

 wires into each of the two binding-posts of simple key No. 1, and connect 

 one wire from each post with one dry cell. Of the other two wires, run 

 one to the anode of the other dry-cell and the other to the femur-clamp. 

 From the remaining plate, the cathode of cell No. 2, run a wire to simple 

 key No. 2, and connect the other post of this key with the muscle-lever. 



