VI , PKEFACE. 



existing in the mind of the author, and may be altered at any time to 

 suit the requirements of more extended observation. In rendering an 

 account, therefore, of the state of knowledge upon any physiological 

 subject, the student should be informed, not only of the results now in 

 our possession, but also of the means of investigation by which they 

 have been attained. He is thus enabled to distinguish between what 

 is positive in physiological doctrines, and what is hypothetical ; and 

 when further discoveries are made, which lead to changes of opinion, 

 he is not confused or disappointed at apparent contradictions between 

 the new views and the old. This method requires a certain amount of 

 detail in the statement of facts; but its advantages are ample compen- 

 sation for the necessary expenditure of time and space. 



The additions and alterations in the text, requisite to present con- 

 cisely the growth of positive physiological knowledge, have resulted, 

 in spite of the author's earnest efforts at condensation, in an increase 

 of fully fifty per cent, in the matter of the work. A change, however, 

 in the typographical arrangement has accommodated these additions 

 without undue enlargement in the bulk of the volume. 



The new chemical notation and nomenclature are introduced into the 

 present edition, as having now so generally taken the place of the old, 

 that no confusion need result from the change. The centigrade system 

 of measurements for length, volume, and weight, is also adopted, these 

 measurements being at present almost universally employed in original 

 physiological investigations and their published accounts. Tempera- 

 tures are given in degrees of the centigrade scale, usually accompanied 

 by the corresponding degrees of Fahrenheit's scale, inclosed in brackets. 



NEW YOKK, September, 1875. 



