34 INTRODUCTION. 



■was still greater, Inasmuch as the tissues were not merely clearly defined 

 and fully and logically treated of, but full account was taken of their 

 physiological functions and morbid alterations. To this great internal 

 progress, the present century has added an ever-increasing perfection 

 of the external aids of the microscope, and a steadily increasing zeal in 

 the investigation of nature, so that it is not to be wondered at, that in 

 its five decades, it has left far behind all that was done in the century 

 and a half of its earlier existence. In the last thirty years particularly, 

 discoveries have so trodden upon one another's heels, that it must be 

 considered truly fortunate that a bond of connection has arisen, and 

 that Microscopical Anatomy has thus escaped the danger of becoming, 

 as in earlier days, lost in minutipe. In the year 1838, in fact, the 

 demonstration by Dr. Th. Schwann, of the originally perfectly identical 

 cellular composition of all animal organisms, and of the origin of their 

 higher structures from these elements, afforded the appropriate concep- 

 tion which united all previous observations, and afforded a clue for 

 further investigations. If Bichat founded histology more theoretically 

 by constructing a system and carrying it out logically, Schwann has, 

 by his investigations, afforded a basis of fact, and has thus won the 

 second laurels in this field. What has been done in this science since 

 Schwann, has been indeed of great importance to physiology and medi- 

 cine, and in part of great value in a purely scientific point of view, 

 inasmuch as a great deal which Schwann only indicated, or shortly 

 adverted to, as the genesis of the cell, the import of the nucleus, the 

 development of the higher tissues, their chemical relations, &c., has re- 

 ceived a further development; but all this has not amounted to a step 

 so greatly in advance as to constitute a new epoch. If, without pre- 

 tensions to prescience, it be permitted to speak of the future, this 

 condition of Histology will last as long as no essential advance is made 

 towards penetrating more deeply into organic structure, and becoming 

 acquainted with those elements, of which that which we at present hold 

 to be simple, is composed. If it be possible that the molecules which 

 constitute cell-membranes, muscular fibrils, axile fibre of nerves, &c., 

 should be discovered, and the laws of their apposition, and of the altera- 

 tions which they undergo in the course of the origin, the growth, and 

 the activity of the present so-called elementary parts, should be made 

 out, then a new era will commence for Histology, and the discoverer 

 of the law of cell genesis, or of a molecular theory/, will be as much or 

 more celebrated than the originator of the doctrine of the composition 

 of all animal tissues out of cells. 



§ 2. In characterizing the present position of Histology and of its 

 objects, Ave must by no means forget that, properly speaking, it con- 

 siders only one of the three aspects which the elementary parts present 



