26 ' INTRODUCTION. 



combination of their constituent materials. So long 1 as they are con- 

 nected with the living- organism, their physical constitution is main- 

 tained by the supply of nutriment from the blood and interstitial 

 fluids. But after this supply is cut off, they still remain for a time 

 sufficiently unaltered to exhibit their specific characters. By this 

 means we learn that the physiological property of a muscular fibre 

 is contractility ; and that there are two kinds of these fibres, the 

 striped and the unstriped, both of which contract under the application 

 of a stimulus, but with different degrees of rapidity. A nerve fibre, 

 on the other hand, has the power of transmitting a stimulus to distant 

 regions, and of calling into activity other parts with which it is con- 

 nected. In certain instances the action and products of special gland- 

 ular tissues may be studied with some success in a similar way. As 

 a general rule, investigations of this kind are most readily carried out 

 in the cold-blooded animals ; because their tissues are the seat of a 

 less rapid alteration than in the warm-blooded classes, and retain their 

 normal properties for a longer time after separation from the body. 



But the functional activity of entire organs, or of an apparatus of 

 associated organs, can be studied only by experimental observation 

 upon the living body. A compound structure produces results in which 

 all its various parts have their share, and which are affected by the 

 manner in which these parts are combined in successive or simultaneous 

 action. Thus every muscular fibre in the walls of the heart has the 

 same simple property of contractility ; but the physical action of the 

 organ, as a whole, is produced by so many contractile fibres, arranged 

 in so complex a form, that it needs a direct inspection of the living 

 heart to show the character, rhythm, and frequency of its pulsations. 

 The glandular organs yield secretions which contain not only the special 

 products of their cells, but also materials supplied from the circu- 

 lating blood ; and this supply varies in quantity and composition 

 according to different nervous and vascular conditions. In the digestive 

 apparatus a number of secretions act, in succession or together, upon 

 the elements of the food, and thus modify the properties derived from 



* their individual composition. These facts make it necessary, in the 

 solution of the most important questions, to study the animal functions 

 by means of observation and experiment during life. 



The progress of physiology at the present day is characterized by 

 the general adoption of methods which yield results in many respects 

 more definite and positive than those formerly attained. This is largely 



I due to the improvements in physics and chemistry, which place at the 

 disposal of the physiologist more effective means of investigation. 

 Many of the phenomena presented by living bodies can now be exam- 

 ined, measured, and recorded by the aid of optical, electrical, photo- 

 graphic and registering instruments, by which our knowledge in regard 

 to them is rendered both more extensive and more precise. We are 

 also enabled by this means to compare the results of different observa- 

 tions, and to reach the important deductions based on the relation of 



