INTRODUCTION. 27 



again be put in operation. The most important facts have often 

 remained long unknown or misunderstood for this reason. The earlier 

 anatomists supposed that, the arteries were tubes for the circulation of 

 air, because they appeared empty when opened after death. It was 

 only when Galen exposed the artery of a living animal, and, opening it 

 between two ligatures, showed it to be full of blood, that the true func- 

 tion of these vessels was ascertained. The lacteal and lymphatic vessels 

 were discovered in the seventeenth century ; but from their small size, 

 and the small amount of fluid contained in them, the circulation in the 

 lymphatic system was thought to be very limited in quantity. Two 

 hundred years afterward, when the experiment was performed of 

 introducing a canula into the thoracic duct of the living animal and 

 continuing the observation while digestion and absorption were going 

 on, the experimenters obtained, in horses and oxen, from fifty to one 

 hundred pounds of lymph and chyle during twenty-four hours ; thus 

 demonstrating the existence of a vital activity much greater than could 

 have been suspected from any examination of the dead bod}^. 



The observation of the physiological actions during life usually re- 

 quires the employment of certain contrivances and manipulations in 

 order to arrive at accurate results. Even the more superficial phe- 

 nomena, such as the changes in the air produced by respiration, can 

 only be studied with precision *by the aid of artificial means for meas- 

 uring and examining the various gases absorbed or discharged. The 

 processes going on in the internal organs are more especially concealed 

 from view, and, therefore, need for their study the use of instruments 

 and operations in order to bring them under observation. It is accord- 

 ingly necessary, in the large majority of cases, to resort to experiment* 

 upon animals in the study of physiology, and all the important 

 knowledge thus far gained has been acquired in this way. But as the 

 physiology of the human species is the main object of our study, and 

 as each different species of animals presents certain peculiarities which 

 distinguish it from others, it becomes essential to know how far we can 

 apply the results derived from experiment upon one species to the 

 physiology of the others, or to that of the human body itself. 



All animals present certain general phenomena in common, namely, 

 those of nutrition, secretion, absorption, movement, and reproduc- 

 tion. The vertebrate animals, to which class man belongs,, are fur- 

 thermore constructed upon the same general plan of organization, and 

 their corresponding organs are evidently the same in character. The 

 different parts of their nervous and vascular systems, their digestive 

 apparatus, their organs of locomotion, of secretion, excretion, and 

 reproduction, have the same relative position, and can be easily recog- 

 nized and compared with each other. The ingredients of their solids 

 and fluids have the same or a similar chemical constitution, and play a 

 corresponding part in the vital processes. The coloring matter of the 

 blood is identical in all of them ; they all absorb oxygen and exhale 

 carbonic acid with more or less activity ; and many or most of their 



