PREFACE. 



It does not seem to me that the value of a text-book is materially en- 

 hanced by elaborate descriptions 'of apparatus and methods, except as 

 they involve principles susceptible of general, physiological application ; 

 nor does it seem profitable to follow out the details of intricate, mathe- 

 matical calculations involved in certain studies, such as physiological 

 optics and acoustics, the results of which are universally accepted. It is 

 sufficient to teach by text-books the science of physiology. The art of 

 investigation and the methods employed in physiological research are to 

 be learned in the laboratory and from special treatises and monographs. 



To those who, by early education and common usage, have long been 

 accustomed to English weights and measures, the metric system frequently 

 fails to convey a definite idea, without a mental reduction to the familiar 

 standard ; but the metric system is now very generally used in scientific 

 works. In the text, the English weights and measures and the Fahren- 

 heit scale of the thermometer have been retained, and their equivalents in 

 the metric system are given in parentheses. In microscopic measure- 

 ments the micromillimetre ( T oV(r of a millimetre, or V-^-^Q ^ an i ncn )? 

 indicated by the Greek letter p, is frequently employed. 



The form and typography of the book have been changed, it is hoped 

 for the better. One new plate and sixty-one new figures have been intro- 

 duced. Two plates and sixty-three figures have been discarded. The old 

 illustrations which remain have been carefully examined and all remedi- 

 able defects have been corrected. For most of the illustrations that have 

 been retained, new electrotypes have been taken from the originals, and 

 thirty cuts have been re-engraved. A few engravings, however, taken 

 from classical authorities, though defective from an artistic point of view, 

 have been retained in their original form. It is due to the publishers to 

 make these statements, and to say that they have spared nothing in the 

 mechanical execution of the work. 



AUSTIN FLINT. 



NEW YORK, August, 1888. 



I HAVE taken advantage of the opportunity afforded by the printing 

 of a second impression of this edition, to introduce a new description of 

 the anatomy of the human ovum, with a lithographic plate, in accord- 

 ance with the very recent studies of the normal, human ovum, by Nagel 

 (1888). These observations appeared after the fourth edition had been 

 completed. I have also introduced three new figures in the text, and 

 have revised the descriptions of fecundation of the ovum and segmenta- 

 tion of the vitellus. 



A. F. 



March, 1889. 



