158 LECTURE VI. 



cherish the conviction that particular substances which 

 find their way into the blood are able to induce particu- 

 lar changes in individual parts of the body by their be- 

 ing taken up into them in virtue of the specific attraction 

 of individual parts for individual substances. We know, 

 for example, that a number of substances are introduced 

 into the body which possess special affinities for the ner- 

 vous system, and that among this number again there 

 are some which stand in a closer connection with certain 

 very definite parts of the nervous system, as for instance 

 with the brain, the spinal cord or sympathetic ganglia, 

 and others again with particular parts of the brain, spi- 

 nal cord, etc. On the other hand, we see that certain 

 materials have some special relation to definite secreting 

 organs ; that they penetrate and pervade them by a kind 

 of elective affinity ; that they are excreted by them ; 

 and that, when there is a too abundant supply of such 

 materials, a state of irritation is produced in these or- 

 gans. But an essential condition in all these cases is, 

 that the parts which are believed to have a particular 

 elective affinity for particular matters, should really exist, 

 for a kidney which loses its epithelium is thereby de- 

 prived of its secreting power. Another condition is that 

 the parts should possess a relation of affinity, for neither 

 a diseased, nor a dead, kidney has any longer the affinity 

 for particular substances which the gland, when living 

 and healthy, possessed. The power of attracting and 

 transforming definite substances can be maintained at 

 most for a short time in an organ, which no longer con- 

 tinues in a really living condition. We are therefore, in 

 the end, always compelled to regard the individual ele- 

 ments as the active agents in these attractions. An 

 hepatic cell can attract certain substances from the blood 

 which flows through the nearest capil'ary vessel, but it 

 must in the first place exist, and in the next be in the 



