H^EMORRHAGIC EXUDATIONS. 161 



of the metamorphoses which the blood undergoes when it stag- 

 nates in vessels which have become occluded (in thrombus), or is 

 effused in individual tissues (as in extravasions and apoplectic 

 centres). Many of the most distinguished inquirers have made 

 this question the subject of the most careful investigations ; the 

 morphological metamorphoses which occur in such sanguineous 

 extravasations have been observed under the microscope, from 

 their earliest origin to their persistent condition at a certain stage 

 of development, or to their final disappearance; yet, notwith- 

 standing all these researches, many of the points already observed 

 remain obscure and wholly inexplicable, the different opinions of 

 inquirers being here more entirely at variance with one another 

 than in any other department in the history of development. The 

 chemical history of these exudations is still more deficient, for 

 here we have actually no observations. Histologists have en- 

 deavoured by the aid of certain micro-chemical means to throw 

 some light on this obscure subject ; but these attempts have either 

 been of no avail whatever, or have yielded very doubtful results, 

 an apparently similar structure behaving differently in different 

 cases under the same reagents. A similar remark may be made in 

 reference to the development of pathological exudations into those 

 abnormal cellular masses which especially characterise cancerous 

 structures, or into those fibrous tissues which we meet with in 

 fibroid tumours. Many young physicists, despairing of the possi- 

 bility of explaining these matters, and the processes on which they 

 depend, by chemical means, have probably shared with us in the 

 sanguine expectation that histology, which had already thrown 

 so much light on the development of normal tissues, would aid 

 our chemical researches; but in these expectations we have 

 demanded more than chemistry is able to accomplish, whilst we 

 have also probably underrated the extreme diversity of these 

 highly complicated vital processes. 



VOL. III. 



