612 LECTURE XXVI. 



organism. These two tissues are intimately related to one another. The 

 latter can act for the former within certain limits. We find here quite 

 extensive assimilation processes, and at the same time there is a consid- 

 erable wearing away. It is seldom that we obtain such a deep insight into 

 the transformations of tissue as in the case of the new formation of bones. 

 To be sure our knowledge in this direction is almost wholly morphological. 

 There has been but little attempt to study this interesting process from a 

 physico-chemical standpoint. Even the fully-developed bone retains, 

 especially as regards its periosteum and medulla, its embryonal charac- 

 ter. From these, new bone material may be formed continually. Thus 

 fractures are healed. Even under normal conditions, however, there is 

 continuously taking place a fusion and new formation of bone-substance. 

 Occasionally we notice the appearance of peculiar cells, the so-called 

 osteoclasts, which cause the dissolution of bony tissue at the place where 

 they appear, while on the other hand we meet with the so-called perforating 

 fibers, which also destroy bones. In pathological conditions often the 

 real disappearance of bony tissue is preceded by decalcification as a primary 

 process. It is interesting to trace the course of all these processes, in 

 order to ascertain how, in each separate case, the breaking down of the 

 bony substance is effected, what agents are active in the process, and why 

 it is necessary that this fusing together of bone and new formation of such 

 tissue are constantly taking place. We must for the present allow all these 

 and similar questions to remain unanswered. We mention these relations 

 especially because we are indisputably justified in assuming that if in a 

 tissue to which we are accustomed to assign a very specific function, and 

 in which we would scarcely expect a priori that important metabolic 

 processes would take place, there is a constant exchange of material, so 

 much more will this be true of all the other tissues which are intimately 

 connected with active metabolism and upon which great demands are 

 placed, and that they will likewise participate in an active interchange of 

 cell-material. 



Such an assumption appears to be particularly justifiable for those 

 organs whose activity, within certain limits, is a continuous one, as is true 

 especially of muscular and nervous tissue. The latter, as we well know, 

 is never at rest. Impulses are constantly passing along the nerve fibers, 

 partly towards the central nervous system, and partly from this to the 

 peripheral organs. The nerves are very intimately connected with the mus- 

 cular tissue. This is evident even from the entire development of the 

 organs, for they early enter into relations with one another. We know 

 also that if the innervation ceases, retrogression soon results, bringing on 

 atrophy, and, in fact, this is evidently due in part to the inactivity of the 

 muscles. Unquestionably, the nerves have to some extent a direct influ- 

 ence upon the metabolic processes in the cells of the muscles themselves, 



