96 THE TISSUES. 



the silver nitrate has taken place, which is easily recognized by the reddish- 

 brown color assumed by the tissues, the mesentery is divided into small 

 pieces, dehydrated first in 95%, then in absolute alcohol, cleared in oil 

 of bergamot, and mounted in balsam. As a rule, the mesothelial cells 

 covering the two surfaces of the mesentery, and the endothelial cells 

 lining the arteries, veins, and capillaries are clearly outlined by the 

 reduced silver nitrate. 



If desired, the tissue may be further stained in hematoxylin (we have 

 used Bohmer's hematoxylin solution) or in a carmin solution after dehy- 

 dration in 95% alcohol, after which they are dehydrated, cleared, and 

 mounted in balsam. In preparations made after this method the endo- 

 thelial cells are outlined by fine lines of dark brown or black color. 



Silver nitrate may also be dissolved in a 2% to 2,% solution of nitric 

 acid, in osmic acid, and various other fluids. Stratified epithelia can 

 also be impregnated with silver nitrate, but only after prolonged immer- 

 sion. They are exposed to sunlight after sectioning on the freezing 

 microtome, or after hardening and imbedding, followed by sectioning. 

 After the reduction of the silver the sections are dehydrated and mounted 

 in balsam. 



Kolossow has devised the following excellent method for demon- 

 strating intercellular bridges : Fine membranes, or even minute frag- 

 ments of previously fixed tissues, are placed for about a quarter of an hour 

 in a 0.5% to \Jc osmic acid (or in a mixture composed of 50 c.c. abso- 

 lute alcohol, 50 c.c. distilled water, 2 c.c. concentrated nitric acid, and 

 i to 2 gm. osmic acid) and then into a 10% aqueous solution of tannin 

 for five minutes, or into a developer consisting of the following : water, 

 450 c.c. ; 85% alcohol, 100 c.c. ; glycerin, 50 c.c. ; purified tannin, 

 30 gm., and pyrogallic acid, 30 gm. In the latter case they are subse- 

 quently rinsed in a weak solution of osmic acid, washed with distilled 

 water, and then carried over into alcohol. 



There are, of course, special methods of fixing and subsequently 

 examining epithelial structures ; these, and the methods of examining 

 gland tissue, will be discussed in the chapters devoted to the various 

 organs. 



B. THE CONNECTIVE TISSUES. 



In the connective tissues, the intercellular substance gives char- 

 acter to the tissue, the cellular elements forming a less conspicuous 

 portion. In their fully developed condition some of the members 

 of the connective-tissue group are only slightly altered from em- 

 bryonic connective tissue. In other members there are developed, 

 in less or greater number, fibers, known as connective-tissue fibers, 

 thus forming reticular connective tissue and the looser and denser 

 forms of fibrous connective tissue. A more marked condensation 

 of the intercellular substance is observed in cartilage; and in bone 

 and dentin a still greater degree of density is obtained by the de- 

 position of calcareous salts in the intercellular matrix. In the dif- 

 ferent types of connective tissue the cellular elements are morpho- 

 logically very similar. The role played by the connective tissues 

 in the economy of the body is largely passive, depending on their 



