ON INJECTING. 253 
commence. ‘The amount of pressure exerted on the piston should 
at first be very slight ; but it will be necessary to increase it as the 
injection proceeds. It is advisable to support the animal in the 
water either with the left hand, or to allow it to rest at the bottom 
of the vessel containing the water. The filling of the spleen should 
be watched carefully, and as soon as it is fully distended, more in- 
jection mass should be prevented from flowing into it by tying a liga- 
ture round its artery. The splenic artery is easily found. It arises as 
a branch of the coeliac axis, and enters the substance of the spleen 
at the hilus on its concave surface. In order to obtain a perfect 
injection of the kidney, it should be drained of all blood by open- 
ing the renal vein. Blood and carmine mass will at first flow out 
together ; but as soon as the carmine flows out freely and unmixed 
~ with blood, the vein should be ligatured, and the vessels allowed to 
fill slowly. The injection may be considered complete when the 
transparent parts about both the upper and lower extremities show 
a reddened and slightly distended appearance. The internal organs, 
when well injected, have a deep red color, and appear as if inflated 
with air. In this operation, the Jungs remain untouched by the 
injection, and they must therefore be injected separately through 
the pulmonary artery either zz sztw, or after they have been excised. 
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, on cooling, 
solidifies, and makes the cutting of extremely thin sections after 
hardening an easy matter. After 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. 
In passing, it may be remarked that some histologists consider it 
preferable to inject the entire animal, not through the aorta, but 
through the carotid arteries ; this operation is, however, much more 
difficult than the preceding one, and there is no real necessity for 
proceeding in this way when the former method yields such 
excellent results. 
HARDENING INJECTED TISSUES. 
Injected tissues must be hardened in spirit. After the first 
day’s immersion, they should be transferred to fresh spirit for two 
more days, and then into fresh spirit again, and kept in this until 
ready for cutting into sections. It is zever necessary to use absolute 
alcohol as a hardening agent; and it is seldom needful to place 
the tissues first in weak spirit and gradually to increase the strength 
up to perfectly anhydrous alcohol. The length of time required 
for hardening depends upon the kind of tissue, its size, and, to a 
certain extent, upon the quantity of spirit used ; the smaller the 
size of the tissue, the more rapidly will the hardening be done. 
