U2 MUSCULAR CONTRACTION. [CH. x. 



carbonic acid, and sarco-lactic acid. After prolonged contraction 

 the muscle consequently acquires an acid reaction. 



These five sets of changes will form the subjects of the 

 following five chapters. 



CHAPTER X. 



CHANGE IN FORM IN A MUSCLE WHEN IT CONTRACTS. 



THOUGH it has been known since the time of Erasistratus 

 (B.C. 304) that a muscle becomes thicker and shorter when it 

 contracts, it was not until the invention of the graphic method 

 by Ludwig and Helmholtz, about fifty years ago, that we pos- 

 sessed any accurate knowledge of this change. The main fact 

 just stated may be seen by simply looking at a contracting 

 muscle, such as the biceps of one's own arm ; but more elaborate 

 apparatus is necessary for studying the various phases in contrac- 

 tion and the different kinds of contraction that may occur. 



These may be readily demonstrated on the ordinary muscle- 

 nerve preparation (gastrocnemius and sciatic nerve) from a frog. 

 By the graphic method, one means that the movement is re- 

 corded by a writing. We shall find that the same method is 

 applied to the heart's movements, respiratory movements, blood 

 pressure, and many other important problems in physiology. The 

 special branch of the graphic method we have now to study is 

 called myography ; the instrument for writing is called a myo- 

 graph ; the writing itself is called a myogram. Put briefly, a 

 myograph consists of a writing point at the end of a lever 

 attached to the muscle, and a writing surface which travels at a 

 uniform rate, on which the writing point inscribes its movement. 



The first thing, however, that is wanted is something to stimu- 

 late the muscle and make it contract ; the stimulus is usually 

 applied to the nerve, and the form of stimulus most frequently 

 employed is electrical. 



The galvanic battery in most common use is the Daniell cell. 

 It consists of a well-amalgamated zinc rod immersed in a cylinder 

 of porous earthenware containing 10 per cent, sulphuric acid; 

 this is contained within a copper vessel (represented as trans- 

 parent for diagrammatic purposes in fig. 133) filled with saturated 



