MUSCULAR CONTRACTION. 539 



in form which the muscular fibres undergo during contraction ; and it is only of late 

 years that this single point has been definitively settled. The idea that the fibres 

 do not shorten, but that they assume a zigzag arrangement during contraction, is not 

 adopted by any modern writers. All are now agreed that in muscular contraction 

 there is an increase in the thickness of the fibre, exactly compensating its diminution in 

 length. This has been repeatedly observed in microscopical examinations, and the only 

 points now to determine are the exact mechanism of this transverse enlargement, its 

 duration, the means by which it may be excited, and its physiological modifications. 

 These questions, within the last few years, have been made the subjects of elaborate 

 investigations by Helmholtz, Du Bois-Reymond, Aeby, Marey, and others ; and, although 

 it is hardly necessary to follow these experimenters through all of their investigations, 

 many points have been developed, particularly by the system of registering the muscular 

 movements, that possess considerable physiological importance. 



One essential condition in the study of the mechanism of muscular contraction is to 

 imitate, in a muscle or a part of a muscle that can be subjected to direct observation, the 

 force that naturally excites it to contraction. The application of electricity to the nerve 

 is beyond all question the most perfect method that can be employed for this purpose. 

 We can in this way excite a single contraction, or, by employing a rapid succession of 

 currents, we can excite either continuous or tetanic action. While the electric current is 

 not identical with the nervous force, it is the best substitute we can employ in experi- 

 ments upon muscular contractility, and it has the advantage of not affecting the physical 

 and chemical integrity of the nervous and muscular tissue. In studying this subject, we 

 shall first follow some of the experiments upon muscular contraction excited artificially, 

 and then apply them, as far as possible, to the strictly physiological actions of muscles. 



There are two classes of phenomena that may be produced by electrical excitation of 

 motor nerves : 1. When the stimulus is applied in the form of a single discharge, it is fol- 

 lowed by a single muscular contraction. 2. Under a rapid succession of discharges, the 

 muscle is thrown into a state of permanent, or tetanic contraction. It will greatly facilitate 

 our comprehension of the subject to study these phenomena separately and successively. 



The muscular contraction produced by a single stimulus applied to the nerve is called 

 by the French, secousse (shock), and by the Germans, Zuckung (convulsion). It will be 

 convenient for us to employ some term that will express this sudden action of the mus- 

 cular fibres, as distinguished from the contraction that takes place on repeated stimula- 

 tion or in continued muscular effort ; and we shall designate a single muscular contraction, 

 then, as spasm, applying the term tetanus, to continued action. 



Spasm produced ~by Artificial Excitation. If an electric discharge, even very feeble, 

 be applied to a motor nerve connected with a fresh muscle, it is followed by a sudden 

 contraction, which is succeeded by a rapid relaxation. Under this stimulation, the muscle 

 shortens by about three-tenths of its entire length. The form of the contraction, as 

 registered by the apparatus of Helmholtz, Marey, and others who have applied the 

 so-called graphic method to the study of muscular action, presents certain interesting 

 peculiarities. We shall give, however, only the general characters of this action, with- 

 out discussing in detail the complicated apparatus employed. 



According to Helmholtz, the -whole period of a single contraction and relaxation of 

 the gastrocnemius muscle of a frog is a little less then one-third of a second. The mus- 

 cles of mammals and birds contract more rapidly, but, with this exception, the essential 

 characters of the contraction are the same. The following are the periods occupied by 

 these different phenomena : 



Interval between stimulation and contraction : 0"'020 



Contraction 0"'180 



Relaxation . 0"'105 



0"-305 



