402 DISEASES OF THE AIR-PASSAGES OF HORSES. 
co-operation between life and structure ceases to exist, and, pre¬ 
viously to death, these changes are frequently very conspicuous. 
When the bond of union between vital power and structure is 
dissolved, a kind of excitement accompanies the change in this 
material relationship necessarily morbid, a species of excitement 
dissimilar to that which is associated with healthy and unchanged 
condition of the living structure. The excitement above alluded 
to is frequently manifested in one or other of the symptoms or 
organs, attended by a proportionate depression of the other vital 
influence of the frame; and, whether it be primarily or secondarily, 
either of the nervous or vascular systems, a great degree of de¬ 
pravation in the vital constituents of the body, and extreme ex¬ 
haustion, are the consequences. This vitiation of the constituents 
of life is seen generally in the last stages of epizootic and adynamic 
fevers. 
This special excitement may arise from a particular exciting 
cause of which it is the effect, and, while it produces this kind of 
irritation, may otherwise injuriously operate upon the vascular and 
organic nervous systems; or it may be consecutive of depression, 
arising as the effect of reaction brought about by changes in the 
circulating fluids from the absorption of hurtful matters or impeded 
excretion of morbid principles: but, whichever may be the case, 
one fact is manifest,—there is a progressive deterioration. The 
functions of secretion and excretion are acted upon by the morbid 
conditions of vital power, and the circulating fluids, together with 
the living solids, are consecutively vitiated. The vascular system 
is excited by the vitiated state of the fluids in circulation, so that 
changes in the one reciprocate upon the other, until the bond of 
union between life and the structures is no longer maintained; 
unless exhaustion of the vascular action, which had been caused 
by the morbid and exciting quality of the fluids circulated in the 
vessels, together with its effects on the secreting and excreting 
organs, should occur, and thus a balance of vital influence be ob¬ 
tained in their favour. Then a new action would commence, and 
their excreting and secreting functions would be resumed, the mor¬ 
bid elements eliminated from the system, followed by a gradual re¬ 
storation of the healthy conditions of the animal. 
There is another complication of bronchitis which I have not yet 
noticed; and my readers will pardon my digression from the usual 
rule of the schools in commencing these papers without the deriva¬ 
tion of the term applied to the disease in question, and proceeding 
through causes, symptohas, progress, &c &c. to the terminations 
and treatment; my intention being that of essaying a description 
of the diseases of the air-passages of the horse in some of their 
most important forms. 
