60 TEXT-BOOK OF PHYSIOLOGY. 



both the passive and active states by corresponding weights, the extension 

 during the latter is progressively greater, until with a given weight the length 

 of the muscle is the same. Under such circumstances, there being no short- 

 ening of the muscle, the force of its contraction manifests itself physically 

 simply as tension. In the successive actions of the muscle represented in 

 the same figure there is to be observed also a combination of a change of 

 length and a change of tension, the ratio of the one to the other being deter- 

 mined by the amount of the supported weights. When the weight is slight 

 in amount, the shortening of the muscle reaches a maximum and the tension 

 a minimum; when the weight is large in amount, the reverse conditions obtain. 



GRAPHIC REPRESENTATION OF THE CHANGE OF SHAPE. 



The contraction of a muscle as it takes place in the living body and 

 under normal physiologic conditions is a complex act persisting for a variable 

 length of time in accordance with the number of stimuli transmitted to it 

 in a given unit of time, and as determined experimentally is the resultant 

 of the fusion of a greater or less number of separate and individual contrac- 

 tions or pulsations. To this enduring contraction the term tetanus has been 

 given. With the aid of appropriate apparatus it has become possible to 

 obtain and record single muscle contractions, to analyze and decompose them 

 into their constituent elements, or to combine them in such a manner as to 

 produce practically a normal physiologic tetanus. As in the experimental 

 study of the phenomena of muscle contraction it frequently becomes neces- 

 sary to remove the muscle from the body of the animal, the muscle of warm- 

 blooded animals are not well adapted for this purpose, owing to the rapid 

 alteration in composition they undergo, with a consequent loss of irritability, 

 when deprived of their normal blood-supply. The excised muscles of cold- 

 blooded animals, such as the frog in which, owing to the relatively slow 

 rate of the nutritive activities, the irritability and contractility endure for 

 a relatively long perio d of time, even though deprived of blood are particu- 

 larly valuable for experimental studies. The muscles generally employed 

 are the gastrocnemius, the sartorius, and the hyoglossus. If kept at a normal 

 temperature and moistened with 0.6 per cent, solution of sodium chlorid, 

 such a muscle will contract for a long period of time on the application of 

 any form of stimulus, but especially the electric. 



Method of Recording a Muscle Contraction. Inasmuch as the 

 changes in the form of a muscle during a single contraction take place 

 with extreme rapidity, their succession, peculiarities, and time relations 

 cannot be determined with any degree of accuracy by the unaided eye. 

 This difficulty can largely be overcome by the employment of the graphic 

 method, the principle of which consists in recording the movements by 

 means of a pen on some appropriate moving and receiving surface. To 

 accomplish this object the muscle is attached at one extremity by a clamp 

 to a firm support, and at the other extremity to a weighted lever, which is, 

 however, sufficiently light to enable it to take up, reproduce, and magnify 

 its movements. The end of the lever provided with a point is applied to 

 a smooth surface, such as glazed paper on a cylinder or plate, covered 

 with lampblack. If the surface is stationary, the contraction is recorded as a 

 vertical line; if it is put in movement at a uniform rate by clockwork, the 



