II. GENERAL PHYSIOLOGY OF MUSCLE AND 



NERVE. 



A. INTRODUCTION. 



IT is seldom that the physical and chemical structure of a tissue, as revealed 

 by the microscope aud the most careful analysis, gives even a suggestion as to 

 its function. No one would conclude from looking at a piece of beef, or even 

 microscopically examining a muscle, that it had once been capable of motion, 

 nor would the most exact statement of its chemical constitution give indication 

 of such a form of activity. The most thorough histological and chemical 

 examination of the bundle of fibres which compose a nerve would fail to sug- 

 gest that a blow upon one end of it would cause to be transmitted to the other 

 end an invisible change capable of exciting to action the cell with which the 

 nerve communicated. To understand such a structure we must first learn the 

 forms of activity of which the tissue is capable, the influences which excite 

 it to action, and the conditions essential to its activity, and then seek an expla- 

 nation of these facts in its physical and chemical structure. 



Contractility. One of the most striking properties of living matter is 

 its power to move and to change its form. At times the movements occur 

 apparently spontaneously, the exciting cause seeming to originate within the 

 living substance, but more often the motions are developed in response to some 

 external influence. This power finds its best expression in muscle-substance. 

 In its resting form a muscle, such as the biceps, is elongated, and when it is 

 excited to action it assumes a more spherical shape, i. e. shortens and thickens, 

 whence it is said to have the property of contractility. It is the shortening, 

 the contraction, of the muscle which enables it to perform its function of 

 moving the parts to which it is attached, as the bones of the arm or leg, and 

 of altering the size of the structures of which it forms a part, as the walls 

 of the heart, intestine, or bladder. Ordinary muscle-substance is arranged 

 in fine threads, each one of which is enveloped in a delicate membrane, the 

 sarcolemma ; these muscle-fibres can be compared to long sausages of micro- 

 scopic proportions. A muscle is composed of a vast number of fibres 

 arranged side by side in bundles, the whole being firmly bound together by 

 connective tissue. Since isolated muscle-fibres have been seen under the 

 microscope to contract, each fibre can be looked upon as containing true muscle- 

 substance and being endowed with contractility. The movements of muscles 

 are the resultant of the combined activity of the many microscopic fibres of 

 which the muscles are composed. 



The rate, extent, strength, and duration of muscular contractions are adapted 



