28 INTRODUCTION. 



a close relation with some common feature of their organization ; 

 while others, on the contrary, are confined to two or three species alone. 

 Thus, the red coloring matter of the blood is identical in color, general 

 composition, optical properties, and physiological action throughout the 

 different groups of quadrupeds, birds, reptiles, and fish ; in all of them 

 the nerve fibres have the same distinctive endowments of motor and 

 sensitive qualities, and the internal reactions are performed by the nerve 

 centres in a similar way. But the power of producing electric shocks 

 exists only in a few species of fish, which resemble in all other respects 

 the fishes which are non-electric. Both the general nature of the more 

 common functions, and the specific character of those which are 

 exceptional, become legitimate sources of knowledge in physiological 

 science. 



The physiology of the human species includes all the more general 

 and fundamental facts common to man and animals, as well as the 

 specific differences peculiar to the human organism. These differences, 

 as a general rule, do not relate to the character of the vital phenomena 

 nor to their mode of production, but only to their quantity or intensity. 

 Thus the animal heat, produced in the living tissues, is generated no 

 doubt by processes of the same kind in the human species as in quad- 

 rupeds ; but the exact temperature of the human body, and its normal 

 variations, are to be determined by direct observation upon man. The 

 msumption of oxygen and the exhalation of carbonic acid take place 

 in essentially the same manner in man as in the higher animals ; but 

 the precise quantity of each, and their numerical relation to other ingre- 

 dients or products of the body, are peculiar to man and must be ascer- 

 tained by special examination. Nearly all the observations, therefore, 

 requiring to be made upon the human subject, relate to matters of detail, 

 / most of the general and fundamental facts being reached by investiga- 

 v tions in the physiology of animals. The exceptions to this rule are 

 mainly connected with certain functions of the nervous system, which 

 are so highly developed in man, as compared with the animals, that their 

 activity becomes different in kind as well as in degree. Thus the 

 faculty of articulate language, which has no existence in animals, has 

 been localized in a particular region of the brain, wholly by means of 

 observations upon man ; and it is probable that the same methods will 

 be requisite in regard to some^ther of the nervous functions. But in 

 most respects the phenomena of human physiology are intimately con- 

 nected with those of the higher animals. 



The study of physiology is naturally divided into several depart- 

 ments or sections, each of which deals with certain special subjects, 

 and is distinguished by the nature of the facts investigated, the 

 methods by which they are examined, and their relation to the vital 

 activity of the whole body. 



The first section is devoted to Physiological Chemistry. It com- 

 prises the study of the chemical ingredients of the living body, their 

 composition and reactions, the source from which they are derived, their 



