THE MECHANICAL RESPONSE. 363 



an elastic structure, but not always the same elastic structure, even 

 when observed under the same physiological conditions. 1 



Influence of mechanical conditions on the course of the 

 excitatory process. — In the preceding paragraph it has been shown 

 that resistance exercises a very material influence on the character of 

 the mechanical response to instantaneous stimulation. Although the 

 question how this influence is exercised has during the last ten years 

 engaged the attention of some of the most skilful experimenters, it has 

 not yet been completely elucidated. What strikes us as remarkable is 

 that the order of the phenomena which characterise the excitatory 

 process in muscle, so long as the muscle contracts freely, gives place to 

 a new order when the muscle is prevented from shortening. Can we so 

 define the fundamental change which is undergone in a muscle when it 

 is excited, as to be able to deduce therefrom an explanation sufficient to 

 account, on physical principles, for both orders ; or must we have 

 recourse to the hypothesis of a secondary excitatory action of a 

 physiological nature, caused by resistance, in which the primary change 

 evoked by the stimulus excites in its turn secondary changes, which add 

 their influence to the change which evoked them ? Each of these views 

 has at the present time strenuous supporters. I propose in the following 

 pages to give an account of the experimental data which appear to be of 

 most value as a basis for the discussion of this question. 



Instead of using the term " resistance," it is convenient to refer to 

 the condition which is produced by resistance to contraction, of an 

 extended elastic body. This condition is commonly designated 

 "tension." We shall afterwards find it convenient to designate as 

 contractile stress that particular form of tension which expresses the 

 effort made by a muscle in response to excitation, to overcome a 

 resistance — whether that of a load which it is able to lift, or of a 

 spring which it is able to bend or stretch. Whether we use the 

 isotonic or the isometric method, we measure contractile stress in 

 grammes. 



Although it has been long recognised that the character of the 

 mechanical response to an instantaneous stimulation depends not only 

 on the strength of the stimulus, but also on the resistance to be 

 overcome, it was not until seventeen years ago that J. v. Kries 

 (now Professor at Freiburg), when working in Ludwig's laboratory, 

 showed experimentally 2 that mechanical conditions affecting the 

 process at any stage in its progress, may alter its character. Referring 

 to this action in a more recent paper, 3 he appears to admit the 

 possibility of its being due to a kind of secondary stimulation — the 

 muscle being re -excited by the very effort which it makes in contracting 

 or striving to contract. A similar view has been advanced by other 

 writers in connection with a doctrine which has since become the 

 subject of much controversy — the doctrine that the excitatory process 

 is made up of two antagonistic component processes, one of which is 

 associated with shortening, the other with relaxation, and that, 

 inasmuch as the contractile component manifestly prevails at the 

 beginning, that of relaxation towards the end, we may associate the two 

 periods of a contraction with these two processes respectively. As it 



a Fick, op. cit., S. 138. 



2 Arch. f. Physiol., Leipzig, 1880, S. 370. 



s Ibid., 1892, S. 3. 



