PREFACE. *X 



which he has introduced in support of these conclu- 

 sions have been collected from the best sources : and 

 where they appeared to him to be either erroneous 

 or defective, he has, in many cases, attempted to 

 correct or supply them by experiments of his own. 

 On every occasion of importance, where his results 

 differed materially from those of preceding authors, 

 he has endeavoured to compensate for the want of 

 experience by a frequent repetition of his experi- 

 ments : and, without regard to any particular opi- 

 nions, he has detailed their results with all the accu- 

 racy in his power. These results, it is hoped, will, 

 in general, authorise the conclusions which have been 

 drawn from them ; but, he is well aware, that many 

 points still remain which require farther elucidation^ 

 and are susceptible of more accurate proof. In the 

 present state of science, however, the physiologist 

 must often be content with probability, nor expect 

 to attain to demonstration but by very gradual ap- 

 proaches to truth. 



oj.-\r < *W"il i i > ' ' * ( * * 



Much attention has likewise been bestowed t<t 

 present the several facts, connected with the sub- 

 ject, in the order in which they have been suc- 

 cessively made known. By thus blending with 

 the recital of facts, the history of their gradual de- 

 velopment, an additional degree of interest is im- 

 parted to them, and an opportunity is, at the same 



