100 HOW TO WORK 



thread seized with forceps, the needle unthreaded and withdrawn, or 

 one end of the thread may be held firmly, while the needle is with- 

 drawn over it in the opposite direction. The thread is now tied 

 over the vessel, so as to include the tip of the pipe only, for if the 

 pipe be tied too far up, there is great danger of its point passing 

 through the delicate coats of the vessel. 



The syringe having been well washed in warm water before com- 

 mencing, its nozzle is plunged beneath the surface of the injecting 

 fluid, the piston moved up and down two or three times, so as to 

 force out the air completely, and the syringe filled with fluid. It is 

 then connected with the pipe, which is firmly held by the finger 

 and thumb of the left hand, with a screwing movement. A little 

 of the injection is, however, first forced into the wide part of the 

 pipe so as to prevent the possibility of any air being included. 



The pipe and syringe being still held by the left hand, the piston 

 is slowly and gently forced down with a slightly screwing movement 

 with the right, care being taken not to distend the vessel so as to 

 endanger rupture of its coats. The handle of the syringe is to be 

 kept uppermost, and the syringe should never be completely emptied, 

 in case of a little air remaining, which would thus be forced into the 

 vessel, fig. 154, pi. XXIII. The injection is now observed running 

 into the smaller vessels in different parts of the organism. 



187. Of the Pressure required for Successful Injection. The 

 requisite amount of pressure for forcing the injection into the finest 

 capillaries may be obtained without using the syringe, i. By employ- 

 ing a long tube, to one end of which is attached a small piece of India- 

 rubber tube furnished with a stop-cock which fits into the injecting 

 pipe. 2. By placing the injecting fluid in a vessel three or four feet 

 above the table and immersing a syphon tube which may be entirely 

 composed of India-rubber or partly of glass. 3. By arranging a glass 

 vessel upon the principle of the wash-bottle, p. 86, pi. XXIII, fig. 153, 

 pressure upon the surface of the liquid being produced by the aid of 

 an India-rubber bottle compressed by a weight or spring, or by 

 pouring mercury into the tube which reaches nearly to the bottom 

 of the flask. The other tube must of course also dip below the 

 surface of the injecting fluid while to its upper free end a piece of 

 India-rubber tubing provided with a stop-cock at its extremity, must 

 be adapted. Other arrangements have also been proposed, but after 

 having tried many different plans, I find that upon the whole, the 

 ordinary injecting syringe is the most successful as well as the cheapest, 

 the most convenient, and the most simple instrument, and it is most 

 easily kept in perfect order. It need scarcely be said that by no 

 mechanical means can such varieties of pressure be obtained as by 



