STRUCTURE OF MUSCULAR TISSUE. 



799 



In seeking to find a law which shall express the general relation of excitation and 

 of sensation, it is to be noted that we possess a knowledge of the two necessary ele- 

 ments, namely, the measure of the constant proportional and the measure o'f the 

 smallest perceptible sensation. The first supplies the unit for dividing the standard 

 into equal parts, the second serves to graduate it, since it determines the zero and 

 renders it tit for use. This may be understood by reference to the following figure: 

 In a line, x> of indefinite length, a zero, 0, is noted. This answers to the minimum 

 of sensation. Starting from the point 0, the line X i divided into equal parts, 1, 2, 

 3, 4, etc , which represent the increments of sensation. From the point 0, a line, 6, 

 of any length is drawn; it represents the minimum stimulus, say the -jLth of a 

 gramme if sensations of pressure are under consideration. Now, since the constant 

 proportional, that is to say, the smallest perceptible difference = j for pressure, a line 



FIG. 281. 



drawn from the point 1 must equal the line b plus J of this line; from the point 2 

 a line equal to the line 1, plus ^ of 1 ; from the point 3 a line equal to 2, plus of 2 ; 

 and from the point 4 a line equal to 3, plus of 3. The third added is thus always 

 an increasing quantity, and as the vertical lines have necessarily the same relation 

 to one another as the weights they represent, it is clear that the differences of length 

 between the line Oand the lines 4, 5, 6, etc , indicate what weight must be employed 

 in order that the perceptible difference should be quadrupled, quintupled, etc. The 

 psycho-physical law which Fechner has deduced from this is that within certain limits 

 sensation increases as the logarithms of the stimulus ; in other words, sensations in- 

 crease as the logarithms, whilst the excitations increase as ordinary numbers. If we 

 represent the stimulus as R and the corresponding sensation as S, while C represents 

 the constant proportional, then S = C log. R when the minimum stimulus is taken 

 = 1. This psycho-physical law only holds good within a certain limit. Beyond this 

 the sensation increases more and more slowly, and soon a maximum excitation is 

 reached, beyond which the relation between stimulus and sensation no longer obtains. 



CHAPTER XV. 



ON THE STRUCTURE AND FUNCTIONS OF MUSCULAR TISSUE. 



1. Structure of Muscular Tissue. 



654. THE capability of executing movements effecting change of place or 

 of form is a power that is so widely distributed through the animal body, 

 that it has been questioned whether every cell or mass of germinal matter 

 does not possess it at some period of its development. The gradual alter- 

 ation of shape exhibited by the white corpuscles of the blood ( 177), by 



