172 THE MICROSCOPE 



allowed to fill slowly. When the transparent parts about the upper 

 and lower extremities show a reddened and slightly distended 

 appearance, the injection may be considered complete. The inter- 

 nal organs, when well injected, have a deep red colour, and appear 

 as if inflated with air. In this operation, the lungs remain un- 

 touched by the injection, and they must therefore be injected 

 separately through the pulmonary artery either in situ, or after 

 excision. In order to render the capillaries of the alveoli perfectly 

 distinct in section, it is usual to distend the air-cells of the lungs 

 by pouring melted cocoa-nut oil down the trachea. The oil soli- 

 difies in cooling and makes the cutting of extremely thin sections 

 after hardening an easy matter. When the injection is completed 

 the open vessels should be tied, and the animal placed in 

 cold water ; half-an-hour afterwards the different parts should be 

 dissected out, and placed in methylated spirit. 



Injecting from Carotid Artery. — A much neater way to per- 

 form the above operation,, though perhaps a little more difficult, is 

 to inject from the carotid artery down towards the heart. First, 

 cut down and expose the large artery and vein of the neck. 

 Either dissect the vein out for a little distance, and then cut it, 

 and hold the cut end over a beaker, or, better, introduce into the 

 vein a small glass tube. Bleeding will freely occur through it. 

 If a clot stops the flow, remove the tube and wash it out or 

 introduce a wire and break it up ; then secure the filled nozzle in 

 the carotid artery. A small portion of the vessel is included 

 between two clamps — flattened, bent wire — and a longitudinal slit 

 made through its walls. After the pipe is well secured, a small 

 quantity of the carmine mass is slowly introduced. The beat of 

 the heart itself will push the mixture on until this first small 

 quantity is seen to colour the gums and eyelids. It is advisable 

 to open the abdominal cavity toward the latter state of the 

 operation, in order that the organs may protrude and become 

 generally filled with the injection mass. 



Blood-vessels in Fish, as the Skate, etc.— I find the following a 

 convenient way : — ^Have ready four of the movable cannulae 

 usually provided with injecting syringe, or if these are not at hand 

 four glass tubes, drawn to the form shown in PI. XIV., Fig. 5. 



