4 PREFACE. 



In 1871, the author published an elaborate series of in- 

 vestigations on the physiological effects of severe and pro- 

 tracted muscular exercise, with special reference to its influ- 

 ence upon the excretion of nitrogen. 1 . If the simple facts 

 resulting from these investigations be accepted as true, they 

 fully confirm the view enunciated in the third volume 

 which is directly opposed to the opinions of many recent 

 observers that muscular exercise largely increases the elimi- 

 nation of urea. 



In the fifth and last volume, an attempt has been made 

 to give a clear account of the physiology of the special 

 senses and generation, a most difficult and delicate under- 

 taking. With regard to the chapters on vision, the author 

 desires to express his acknowledgments, for most valuable 

 aid, to his colleague, Professor H. D. E"oyes, who carefully 

 overlooked this portion of the work and made many impor- 

 tant corrections and suggestions. If this section be found 

 fully up to the present day in ophthalmology, it is largely 

 due to the assistance rendered by Professor Noyes. 



Finally, as regards the last, as well as the former vol- 

 umes, the author can only say that he has spared neither 

 time nor labor in their preparation ; and the imperfections 

 in their execution have been due to deficiency in ability and 

 opportunity. He indulges the hope, however, that he has 

 written a book which may assist his fellow-workers, and in- 

 terest, not only the student and practitioner of medicine, but 

 some others who desire to keep pace with the progress of 

 Natural Science. 



1 New York Medical Journal, June, 1871. 

 NEW YORK, Nay, 1874. 



