6 MANUAL OF HISTOLOGY. 



remembered as the discoverer of isomorphism. Among those who have 

 lived in our own day, LieUg (1803-1873) may be said to have taken the 

 place of the last-named Swedish philosopher. By his labours in the field 

 of organic combination he has made himself celebrated, and has obtained, 

 also in a wider circle than any other, a recognition of his genius by his 

 imperishable popular essays on the science. He may be looked upon as 

 the founder of the physiological chemistry and elementary analysis of the 

 present day. Another important step towards an insight into the origin 

 of organic substances in the body was made in 1823 by Wohler, Liebig's 

 talented coadjutor, through his well-known discovery of the composition 

 of urea, 



5. 



Study of the nature of the substances occurring in the animal economy, 

 their properties, constitution, transformations, &c. constitutes what is 

 termed " ZOOCHEMISTRY." The application of zoochemical facts to the elu- 

 cidation of processes taking place in the system, the contemplation of the 

 chemical features of life and significance which the elements of composition 

 have in the same, includes, if not all, yet the chief objects of "PHYSIOLOGICAL 

 CHEMISTRY." That both these branches of study could only be carried on 

 subsequent to the arrival at a certain degree of maturity of chemical science, 

 is perfectly obvious, as has been already remarked, and requires no farther 

 comment. 



Again, the special application of the facts of physiological and zoochemis- 

 try to the tissues composing our frame, constitutes what is termed " HISTO- 

 CHEMISTRY." Its sphere lies in the consideration of the chemical con- 

 stitution of the "structural elements," and consequently also of the tissues. 

 It is engaged with the substances occurring in the latter, their introduction, 

 origin, and the significance they possess in the life of the/orw and tissue- 

 elements ; it traces their metamorphosis, decomposition, and elimination. 



At present we can only boast of a very rudimentary histochemistry. 

 In fact, we are met at all points by the most discouraging difficulties in 

 this branch of study, owing to the nature of the subject to be dealt with. 

 Compared with the extraordinary accuracy of anatomical analysis, through 

 the aid of the microscope of the present day, the means at the disposal of 

 the chemist for the separation of the unstable constituents of the tissues 

 appear coarse and rude. While the histologist is able, for instance, to dis- 

 tinguish with ease, in the most ordinary form-element the cell, envelope, 

 contents, nucleus, and nucleolus, the chemist is still unable to bring these 

 several parts within the grasp of his analysis. Further, it is a rare thing 

 with him to succeed in the analysis even of similar structural elements for 

 themselves, even setting aside their ultimate composition ; for, owing to 

 the complex nature of most tissues, he has to deal with a mixture of several 

 kinds of form-elements, which cannot be separated by chemical means. 



After what we have just seen, too much must not be expected from the 

 histochemistry of the present day. And yet we need not forget, in the 

 contemplation of its necessary deficiencies, how much this special branch 

 of science has produced. We must remember, farther, that without a 

 knowledge of composition, true scientific study of histology is impossible, 

 and the latter is in danger of degenerating into a mere toying with details 

 of form. And as histochemistry, on the one hand, can only be based on 

 a clear insight into the minute anatomical relations of the tissues, so does 

 it form, on the other hand, the indispensable complement to histology. 



