APPENDIX. 487 



d. While injecting, a steady pressure should be main- 

 tained. Should resistance to this be offered, the 

 operation must be interrupted until the cause of 

 obstruction shall have been ascertained and, if pos- 

 sible, removed. 



Obstruction generally arises from one or more of 

 the undermentioned causes. 



i. The presence of clotted blood in the smaller 

 vessels. This source of difficulty is usually 

 fatal to success. 



ii. Resistance offered in the capillary systems, 

 often resulting from the forcing back of the 

 blood upon the great vessels. This may ge- 

 nerally be overcome by puncturing a large 

 vessel as far removed from the point of opera- 

 tion as possible (i. e. if injecting the arteries 

 puncture one of the larger veins, or vice versd). 



iii. A too rapid cooling of the gelatine within the 

 smaller vessels and capillary systems. To over- 

 come this, increase the temperature of the water 

 in which the animal is immersed. 



e. Complete injection of a given capillary system ul- 

 timately results in visible distension of the organ 

 concerned ; should enlargement such as this become 

 suddenly obvious the operation must be terminated, 

 otherwise rupture and extravasation will ensue. 



f. On withdrawal of the syringe the end of the india- 

 rubber tube attached to the cannula should be either 

 plugged with a glass-rod or other convenient stopper, 

 or clamped ; and the whole should be placed in cold 

 water until the gelatine shall have set. 



g. Animals or tissues thus injected should never be 

 placed at once into strong alcohol, but into spirit 

 of increasing strength, from 50 per cent, upwards as 

 directed at Appendix E. 



