ELEMENTS OF STRUCTURE. 



But it is otherwise in the construction of some tissues, in which the 

 various cells commence to cohere 

 and fuse more and more into one 

 another, losing eventually, in many 

 instances, their independence com- 

 pletely. By such series of meta- 

 morphoses (and they occur widely 

 throughout the animal system, and 

 are therefore of the greatest impor- 

 tance) networks of cells, tubes, fibres, 

 and such like, may be formed. The 

 changes in question are of the most 

 various kind, but cannot be de- 

 scribed in all cases with desirable 

 accuracy. It will suffice, however, 

 to take a few of the better known 

 as examples. 



The finest tubes in the circulatory 

 system, namely, the capillaries (fig. 

 99, A, a b, and B, a), are found by 

 ordinary examination to be made 

 up of a delicate transparent mem- 

 brane, in which nuclei are imbedded 

 at intervals. Until a few years ago, 

 this was generally thought to be the 

 entire structure of the capillary 

 tubes, whose development was ex- 

 plained in the following manner. 

 Formative cells were supposed to fuse 

 together, the cavities of the cells to 

 become the lumen of the tube by 

 opening into one another, and the 

 walls of the cells with their nuclei 

 to supply the delicate transparent 

 nucleated membrane of the vessel. 



From the German investigators Hoyer, Auerbach, Eberth, and Aeby 

 we have recently learned, however, the true 

 structure of the capillaries, and the incor- 

 rectness of the former views entertained 

 with regard to them. 



By treatment with a solution of nitrate 

 of silver, namely, this fine membrane may 

 frequently be resolved into extremely thin 

 nucleated formative cells of considerable 

 size, terminating in laps and processes (fig. 

 100), by which the cells adhere to one an- 

 other at their edges, and, taking the hol- 

 lowed form of the lumen, thus produce the 

 vessel. It is the action of light on the 

 silver at the junction of the elements 

 which makes their boundaries visible. 

 Thus we see that the lumen has not had its 

 origin in the cavities of coalescing cells, but is rather an intercellular space. 



Fig. 99. Small blood-vessels from tlie pia mater 

 of the human brain. A, & twig, c, terminates 

 above in two delicate capillaries, a b; B, & simi- 

 lar vessel, with capillaries, a; (7, a stronger twig, 

 with longitudinal and transverse nuclei. 



Fig. 100. Capillary vessel from the lung 

 of a frog, after treatment with dilute 

 solution of nitrate of silver, a, nuclei; 

 b, boundaries of the cells. 



