XXXVI INTRODUCTION. 



tion of these, the organ, after being injected, is immersed in 

 some reagent which destroys the tissue, whilst it leaves the 

 injected mass intact. The form of the vascular network is 

 thus obtained in coloured wax, and such preparations can be 

 put up in various ways under glass and in frames. Injections 

 made with transparent solutions are now very common. A 

 canula is inserted into an artery, and the fluid allowed to 

 discharge itself by a vein. The dissolved material penetrates 

 the capillaries whilst the coarsely granular pigment is stopped 

 in the larger vessels. In such preparations it is obvious that 

 no difference can be seen between the arteries and the veins ; 

 but, in this condition, they are not fit for microscopic exami- 

 nation. It is still requisite to harden them by freezing mix- 

 tures, or by means of alcohol, and then to make fine sections. 

 In these injections it is always requisite that a certain fulness 

 and tension should be given to the vessels ; their forms then 

 assume greater definition, and are generally more similar to 

 their natural condition. On this account it is advantageous to 

 dissolve the colouring matter in something which will readily 

 coagulate, and which consequently affords all the advantages 

 of a hardened tissue. Fine gelatine is usually employed, and 

 is dissolved in water over a water bath, the colouring matter 

 already in solution being then added, and the warm mass intro- 

 duced into a Woulf s bottle, which again must be immersed in 

 a warm water bath. The injection with gelatine is sufficiently 

 tedious if required to be done thoroughly, as the mass stiffens 

 too easily. The organ to be injected should therefore be 

 brought into a warm room, and, where practicable, placed 

 over a water bath which is adjacent to the former one. 



The colouring matters usually employed are Prussian blue 

 and carmine ; the latter not in a state of complete solution, 

 but partly precipitated by the addition of a little weak acid 

 from its alkaline solution. Thiersch, whose transparent injec- 

 tions are perfect models of this kind of art, uses a transparent 

 green and yellow. He obtains the former from chromate of 

 potash and nitrate of lead, the latter from a mixture of this 

 with blue. When the injection with gelatine is completed, 

 the open vessels must be tied, and the organ introduced or sus- 

 pended in alcohol contained in a wide-necked bottle, pressure 



