126 THE MICROSCOPE. 



capillaries (as in the liver), the injection may be effected 

 satisfactorily immediately after the death of the animal. 



" The steps of the process are very similar in making the 

 opaque injections, except that when size is employed, the 

 specimen must be placed in warm water until warm 

 through, otherwise the size will solidify in the smaller 

 vessels, and the further flow of the injecting fluid will be 

 prevented. Soaking for many hours is sometimes neces- 

 sary for warming a large preparation thoroughly, and it is 

 desirable to change the water frequently. The size must 

 also be kept warm, strained immediately before use, and 

 well stirred up each time the syringe is filled. 



" In the first place, the following instruments must be 

 conveniently arranged : 



" The syringe thoroughly clean and in working order, 

 with pipes, stopcock, and corks. 



" One or two scalpels. 



" Two or three pair of sharp scissors. 



" Dissecting forceps. 



" Bull's-nose forceps, for stopping up any vessel through 

 which the injection may escape accidentally. 



" Curved needle, threaded with silk or thread, the thick- 

 ness of the latter depending upon the size of the vessel to 

 be tied. 



" Wash-bottle. Injecting fluid in a small vessel. 



" I will commence with the frog. An incision is made 

 through the skin, and the sternum divided in the middle 

 line with a pair of strong scissors ; the two sides may 

 easily be separated, and the heart is exposed. Next the 

 sac in which the heart is contained (pericardium) is opened 

 with scissors and the fleshy part of the heart seized with 

 the forceps ; a small opening is made near its lower part, 

 and a considerable quantity of blood escapes from the 

 wound this is washed away carefully by the wash-bottle. 

 Into the opening the tip of the heart being still held 

 firmly by the forceps, a pipe is inserted and directed 

 upwards towards the base of the heart to the point where 

 the artery is seen to be connected with the muscular sub- 

 stance. Before I insert the pipe, however, I draw up 

 a little of the injecting fluid so as to fill it, for if this were 

 not done, when I began to inject, the air contained in the 



