BEl T. 2l6.] 



CAriLLARY NETWORKS. 



499 



Fig. 204. 





ception, possesses a certain number of elongated cell-nuclei, of 

 0-003'" to 0-004'" m s ^ ze > which are sometimes disposed at wider 

 intervals, and then generally alternate on the two sides of the 

 vessel, but are sometimes nearer, or even quite close together. In 

 thin vessels, these nuclei have their seat upon the inner side ; but in 

 thicker ones they are placed in the substance of their wall, in such a 

 manner that they not unfrcqucntly occasion projections of the walls 

 outwards. The diameter of the capillaries, in man, varies between 

 0-002'" and o - oo6'", and they may, for the facility of description, 

 be again divided into a finer sort, measuring from 0*002'" to 0-003'" 

 in diameter, and having scanty nuclei and thin walls ; and into a 

 coarser kind, 0-004'" to o - oo6'", with thicker coats and numerous 

 nuclei; the two groups, it is to be understood, are not separated 

 by any natural limit. 



By the union of the capillaries arise the capillary networks, 

 retia capillaria, which have 

 been alreadv treated of in de- 

 tail in the description of the 

 several organs and tissues, and 

 will, therefore, only be shortly 

 spoken of here in general terms. 

 Their forms, in spite of con- 

 siderable variations, are still 

 pretty constant for the several 

 organs, and, according to their 

 uniformity or peculiarity, are 

 more or less characteristic. 

 The various forms depend, in 

 great measure, upon the ar- 

 rangement of the elementary 

 parts, but also upon the energy 

 of the functions. With regard 

 to the former, there are, in 

 many organs, certain ultimate 

 tissues, into which vessels never 

 penetrate, such as the trans- 

 versely striated muscular fibres, 

 the bundles of connective tissue, 

 the nerve-tubes, cells of all 

 kinds, and gland-vesicles- and 

 the course of the capillaries 

 is definitivelv determined ac- 



Finest vessels of the arterial si.le. 1. Smallest 

 artery; 2. transition vessel; 3. coarser capil- 

 laries; 4. finer capillaries, a. Structureless mem- 

 brane, still with some nuclei, representative of 

 the tunica udventitia ; 6. nuclei of the muscular 

 fibre-cells; c. nuclei within the small artery, 

 perhaps appertaining to an epithelium; '/.nuclei 

 of the capillaries of the transition vessels. From 

 the human brain ; magnified 300 times. 



K K 2 



