210 
DEPARTMENT OF SURGERY. 
aqueous humor, but this activity (secreting) of the membrane 
is more marked near the vascular periphery of both chambers ; 
the part lining the ciliary processes produces the greatest quan¬ 
tity. The lymph, vessels, which normally carry off the aqueous 
humor, are located in the spaces of Fontana, which communi¬ 
cate with the canal of Schlemm. (Fig. 48-5.) 
The transparency of the cornea is maintained by the nor¬ 
mal apposition of these layers, and a perfect physiological 
function of vessels carrying nutrition to it, and its absorbing 
system. Either of these conditions depend to a certain extent 
upon intraocular pressure; the position or relation of these 
structures can be changed by an abnormal pressure, and increased 
pressure may prevent the access of nutrient juices into the cor¬ 
nea, or interfere with the absorbing system ( lymphatics ) ; while 
a diminution of pressure may increase the amount of juices and 
allow the accumulation of lymph through a lack of tonicity, in 
lymph vessels, any deviation may cause disease of the cornea, 
impair the functional activity, its circulatory apparatus, or cause 
it to lose its transparency ; besides these conditions resulting 
from interocular, the bulging of the cornea is the one to which 
paracentesis is most applicable, because the other conditions 
often are difficult to attribute to the real exciting cause, and it 
is not advisable to experiment much with the organ of vision 
of valuable animals. 
Operation .—The operation is not a difficult one, but one that 
must be performed aseptically. The anterior chamber can be 
punctured with a probe, hollow needle, or small trocar. To 
empty the anterior chamber a probe is sufficient, although some 
oculists recommend the trocar for the removal of pus ( hypopyon ) 
from the chamber ; it is also recommended for tapping posterior 
chamber. 
Before operating the patient must be properly secured ; with 
some patients a local anaesthetic will answer, but if it cannot be 
properly controlled, a general anaesthetic must be substituted. 
The eyelids can be separated with the fingers, but if the pos¬ 
terior chamber is to be tapped, they can be separated with an 
eye speculum. (Fig. 50-A). The eyeball is fixed with the 
thumb and forefinger of the left hand ; the instrument (Fig 50- 
B. C.) is entered at the corneo-scleral margin, at right angle 
with the surface (Fig. 49-a) and as the instrument is inserted 
the direction is changed so that the iris and lens is not injured; 
the instrument is removed gradually, allowing the aqueous 
humor to escape slowly. If the contents of the chamber is 
