460 PRINCIPLES OF GENERAL PHYSIOLOGY 



Details of the movements of plants and the interesting facts in connection 

 therewith will be found in Pringsheim's book (1912). 



THE GRAPHIC METHOD 



In the methods of analysing the forms of muscular contraction, we have, for 

 the first time in the present book, come across a systematic use of the graphic 

 method, so that a few words on the subject may not be out of place here. 



Any representation of the relation of two phenomena to one another by 

 drawing a curve on squared paper is, of course, a "graph." But the name 

 "graphic method" in physiology is especially used to refer to cases where the 

 curve is drawn by the apparatus used to observe the phenomenon. The abscissa; 

 are nearly always time, the curve being made on a moving surface. 



This surface may be of glazed paper or of glass, in either case smoked by a flat 

 flame, supplied with gas which has passed over cotton wool wet with benzene. In 

 this way, sufficient smoke can be deposited without burning the paper, if this is 

 moved rapidly through the flame by rotating the drum or other surface on which 

 the paper is fixed. 



Although the present book is not primarily intended as a laboratory guide, it may be useful 

 to mention that, when the force moving the point which scratches away the smoke is very 

 small, such as that of the frog's auricle, it will be found very important to have as little 

 friction as possible between the paper and the point. In such cases, the form of tracing 

 point devised by mj'self (1912, 1) will be found useful. Bose (1913) has worked out a delicate 

 method for tracing the movements of such structures as those of the leaves of sensitive plants, 

 which have very little force. The friction of the tracing point on the recording surface is 

 practically abolished, by causing it to vibrate rapidly in a plane perpendicular to the surface, 

 so that contact takes place only momentarily. This vibration is effected by an electro- 

 magnetic arrangement. 



The nature of the varnish used for fixing the curves is not a matter of indifference. 

 Ten per cent, shellac in 90 per cent, alcohol, or ordinary white hard varnish, diluted with 

 an equal volume of alcohol, serves well, but, in either case, it is better to add a few cubic 

 centimetres of castor oil to the litre to prevent brittleness when dry. The tracing should 

 be quite dry when the varnish is applied, by drawing the paper through it, and it should 

 be allowed to harden in a dry atmosphere. Further information is to be found in the 

 article by Frank (1911). 



Photographic methods have many advantages. A beam of bright light is 

 reflected from a mirror attached to the moving part of the apparatus ; any inertia 

 can thus be avoided. The beam is passed through a slit and forms a point of light 

 on a moving sensitive surface, paper or plate, behind the slit. Sometimes the 

 shadow of a small moving part is projected on to the slit by means of a 

 microscope, as in the " string " galvanometer. Details of the various methods will 

 be found in the article by Garten (1911). 



An excellent form of photographic registration apparatus for paper or plates is that made 

 by the Cambridge Scientific Instrument Company for use with the "string" galvanometer, 

 but is available for any form of photographic method. 



SUMMARY 



In the animal, there are certain tissues, known as muscular, which have the 

 function of causing movement of parts relative to one another or, if the ends are 

 so fixed that no change of place can occur, a state of tension is developed. 



There are two chief classes of contractile tissue the cross-striated, skeletal, 

 which is dependent on impulses from the central nervous system to set it into 

 activity, and the smooth, or involuntary, muscle also under the control of the 

 central nervous system, but capable of exerting an automatic, tonic contraction 

 or a rhythmic series of contractions. The latter class of muscular tissues, although 

 subject to reflexes, are not under voluntary control. The muscular coat of the 

 arterioles and the heart are examples of this class. 



The rate of contraction of smooth muscle is slower than that of skeletal 

 muscle ; but numerous varieties occur in both, so that the extreme cases do not 

 greatly differ. 



