50 HIS, ON THE CORNEA. 
other points of view in which they are eminently worthy of 
attention. 
The cornea is particularly well fitted for this class of inves- 
tigations, from its transparency, its thinness, and that marked 
peculiarity of structure which enables abnormal alterations 
to be so easily detected. At the same time the absence of 
any vascular network throughout the greater part of its sub- 
stance has the great advantage of freeing the mind of the 
inquirer into the essence of the earlier stages of its alteration, 
from the thraldom of that “action of the vessels” hypo- 
thesis, so long the deus ex machind of British physiologists 
and pathologists. 
The structure of the cornea has been a matter of much 
controversy among histologists. It has been affirmed to be 
composed of superimposed lamelle, the number of such 
lamelle varying, according to the describer, from four to two 
hundred. Some have supposed these lamellz to be glued 
together by an invisible cement; others have been led to 
conceive that interspaces, the “corneal tubes,” normally 
exist between the lamelle ; another school of observers has 
denied the existence of lamelle altogether, maintaining that 
the cornea is composed of interlaced fibres. Some have 
asserted that the cornea is fitted into the sclerotic as a watch- 
glass into its frame; others, on the contrary, affirm that the 
tissue of the one is continuous with that of the other. Be- 
fore giving an account of Dr, His’s view we must describe 
his mode of operation—an essential element in questions of 
this kind. 
The cornea may be examined either with or without pre- 
vious preparation. In the latter case the one thing needful 
is, that the sections, whether vertical or horizontal, be made 
with an extremely sharp scalpel, and at one draw, without 
sawing. A good vertical section should not be broader than 
the thickness of the cornea, and should be examined without 
a covering glass. Without these precautions the natural 
appearance undergoes great and immediate alteration. 
Pure and colourless dilute pyroligneous acid (Dr. His 
unfortunately does not give the precise strength) is the best 
preservative and hardening agent ; but the preparations must 
not be exposed to the sun or they become brown and opaque. 
Sections of extreme thinness can readily be cut from cornez 
hardened in this acid. 
Dilute hydrochloric and sulphuric acids (in equal propor- 
tions of acid and water) are of great service, asmuch as 
they destroy the intercellular substance, and leave the cor- 
neal cells, vessels and nerves. A drop or two of either of 
