28 INTRODUCTION. 



secretions and excretions have the same physiological character. The 

 whole value of physiological experiment, as applied to different species, 

 depends upon this general resemblance between them, both of structure 

 and function. 



On the other hand, the differences between species of vertebrate ani- 

 mals consist only in the relative size and development of particular 

 parts, and consequently in the relative importance of particular func- 

 tions. The intestine, for example, is longer and more complicated in 

 the herbivorous animals, shorter and simpler in the carnivora. The 

 muscles of the external ear are slightly developed and powerless in the 

 human subject, large and active in many of the inferior species. Fish 

 and reptiles produce but little animal heat, and are, therefore, called 

 cold-blooded animals ; birds and quadrupeds generate it in abundance, 

 and are therefore called warm-blooded. The differences between them 

 are, therefore, almost invariably differences in degree and not in kind. 



Consequently the simple and direct result of an experiment in different 

 animals is the same, or varies only in degree. If we deprive an animal 

 of oxygen, whatever the species may be, it produces death invariably and 

 in the same way, because in all this element is essential to the nourish- 

 ment of the tissues. But death will take place rapidly in birds or 

 quadrupeds, more slowly in reptiles, because the vital changes are more 

 active in the former than in the latter. Division of the spinal cord in 

 all cases produces immediate paralysis of sensation and voluntary 

 motion in the parts below, showing that the sensitive and motor fibres 

 follow in all the same route and possess the same nervous endowments. 

 Experiments accordingly of the same kind, performed upon different 

 animals, have a direct result which is the same in character. 



But experiments have often also certain indirect or secondary results, 

 dependent upon the relative importance of associated organs, and these 

 vary considerably in different kinds of animals. Thus division or dis- 

 ease of the facial nerve in all instances causes a direct paralysis of the 

 muscles of the face. In the human subject this produces only a loss of 

 expression, with some inconvenience in the retention of fluids by the 

 mouth. But in the horse it is followed by a partial suffocation, because 

 in him the expansion of the nostrils is an important part of the move- 

 ments of respiration. While the direct effect of an experiment, there- 

 fore, is always the same, its indirect effect varies with the comparative 

 development of different parts. It is evident, however, that this varia- 

 tion does not impair the value of experiment as a means of study, but, 

 on the contrary, enlarges its usefulness and leads to the acquisition of 

 greater knowledge by its means. 



The physiological actions of living beings are, of course, dependent 

 upon natural causes, and are to be studied in a similar manner with 

 other natural phenomena, such as those of magnetism, gravitation, 

 chemical affinity, and the like. In all these cases, we observe the 

 character of the phenomenon, the conditions upon which it depends, 

 the mechanism of its production, and the quantities of force or material 



