106 Histology and Embryology. By C. S. Minot. 
in the unstriated form. Different as is muscle in appearance from 
cells, yet it originates from them, and is in fact formed of meta- 
morphosed cells, by a series of changes all as great as those which 
produce bone. 
We have still to notice a very important class of procedures, 
namely, injections. In the higher animals we find two distinct sets 
of vessels ramifying through the whole body : one of these is the 
system of blood-vessels, the other the lymphatic system. As is 
well known to all, the blood-vessels branch out into very fine tubes 
that form a complicated network in every part of the body, so fine 
that it can only be followed when the tubes or capillaries have been 
artificially filled with a coloured matter. The same is true of the 
lymph-vessels, but to an even greater extent. Many of the 
structures of the body are permeated by connective tissue, and 
in this tissue there are numerous cavities filled with fluid ; they 
are in communication with very delicate tubes, the lymphatic 
capillaries, which soon unite into larger canals, and these form 
branches which gradually join together and lead to the thoracic 
duct or main stem, which empties into the veins just before they 
open into the heart. The branches of this tubular system are 
provided with valves so arranged that the liquid contained in the 
tubes can only pass upward or towards the main stem. Now when 
any motion takes place, some of the liquid in the cavities of the 
connective tissue is pressed into the lymphatics and so slowly 
driven onwards into the heart. To counterbalance this loss of 
intercellular fluid, certain constituents of the blood exude through 
the walls of the capillaries and keep up the supply. There is, 
therefore, a double circulation : one within the blood-vessels, and 
another from the walls of the capillaries through the lymphatics. 
The liquid in both circulations is ultimately returned to the heart. 
Different methods have to be employed for injecting the two 
systems. In the case of the blood-vessels a rather large syringe 
may be used, provided with a point small enough to pass into the 
artery of the part to be injected. The artery should be carefully 
laid bare and cut about half-way through ; the point of the syringe, 
which must be previously filled with the injection mass, is pushed 
into the artery and firmly tied in place. In many cases it is quite 
sufficient to inject a cold saturated solution of Prussian blue in 
water, or when more perfect preparations are wanted, a little 
gelatine may be added ; in this case, however, there arises the 
inconvenience that both the injection mass and the organ to be 
injected have to be kept warm while the operation is going on, 
otherwise the gelatine solidifies. 
To prepare a “warm” injection mass, the following method is, 
perhaps, the best. A solution of Prussian blue is necessary ; this 
the histologist must make for himself. To do this, take a con- 
