PREFACE. v 



objection, that too many disputed matters have been dis- 

 cussed : this was deliberately done as the result of an experi- 

 ence in teaching Physiology which now extends over more 

 than ten years. It would have been comparatively easy to 

 slip over things still uncertain and subjects as yet unin- 

 vestigated, and to represent our knowledge of the workings 

 of the animal body as neatly rounded off at all its contours 

 and complete in all its details totus, teres, et rotundus. 

 But by so doing no adequate idea of the present state of 

 physiological science would have been conveyed; in many 

 directions it is much farther travelled and more completely 

 known than in others; and, as ever, exactly the most in- 

 teresting points are those which lie on the boundary 

 between what we know and what we hope to know. In 

 gross Anatomy there are now but few points calling 

 for a suspension of judgment; with respect to Micro- 

 scopic Anatomy there are more; but a treatise on Physiology 

 which would pass by, unmentioned, all things not known 

 hut sought, would convey an utterly unfaithful and untrue 

 idea. Physiology has not finished its course. It is not cut 

 and dried, and ready to be laid aside for reference like a 

 specimen in an Herbarium, but is comparable rather to a 

 living, growing plant, with some stout and useful branches 

 well raised into the light, others but part grown, and 

 many still represented by unfolded buds. To the teacher, 

 moreover, no pupil is more discouraging than the one who 

 thinks there is nothing to learn; and the boy who has 

 "finished" Latin and " done" Geometry finds sometimes his 

 counterpart in the lad who has " gone through" Physiology. 

 For this unfortunate state of mind many Text-books are, I 

 believe, much to blame: difficulties are too often ignored, or 

 opening vistas of knowledge resolutely kept out of view: tho 

 forbidden regions may be, it is true, too rough for the young 

 student to be guided through, or as yet pathless for the 

 pioneers of thought; but the opportunity to arouse the re- 

 ceptive mental attitude apt to be produced by the recogni- 

 tion of the fact that much more still remains to be learned 

 to excite the exercise of the reasoning faculties upon dis- 

 Duted matters and. in some of the better minds, to arouse 



