HEREDITARY DISEASES OF HORSES. 
529 
in animals of more perfect conformation. But it appears to 
us that internal and constitutional hereditary diseases also 
depend upon the altered conformation or texture of the parts 
specially affected, or upon some disturbance of the relation 
which should subsist between the different constituents of 
these parts. This abnormal state of the internal parts is 
seldom within the limits of our means of observation or 
investigation, but its existence in animals having a hereditary 
predisposition to disease cannot, we think, be doubted, as we 
shall now endeavour to show. The ground of our reasoning 
rests chiefly on the analogy which subsists in all respects 
between external and internal parts. The same law which 
regulates the hereditary transmission of form, texture, and 
relation of external and visible parts, also operates with equal 
force in regard to the form, texture, and relations betwixt the 
components of parts internal, and, it may be, inaccessible to 
ordinary powers of investigation. Then, if, as we have 
shown, external hereditary diseases, such as lamenesses, are 
traceable to external hereditary peculiarities of conformation, 
we do not think it pushing our analogy too far to assert that, 
in like manner, internal hereditary diseases must, in great 
part at least, depend upon some inherent hereditary peculia- 
rity of the internal parts affected. The following remarks 
will, we think, tend to support our hypothesis. Particular 
conditions of the blood often become hereditary, and, if an 
excess of the red globules of the blood be hereditary, the 
disease of plethora to which that excess gives rise will also 
become hereditary. We have a striking example of this in 
many of the improved breeds of cattle, in which is conjoined 
a remarkable excess of the red globules of the blood with a 
highly plethoric habit of body. If the eye be predisposed to 
deep-seated ophthalmia, a slight exposure to cold, or even an 
error in diet, will be sufficient to induce the disease. But 
before an acute attack there is seldom noticeable in the eye 
any alteration of texture or of function indicating the existence 
of such a tendency. That such a tendency does, however, 
exist there can be no doubt, and we think that it must con- 
sist in an altered condition of some of the deeply-seated parts 
of the eye. Our conclusion is, therefore, that every here- 
ditary disease depends upon some hereditary abnormal condi- 
tion predisposing to that disease. This abnormal condition 
may be either local or general. It may affect the form, 
structure, texture, quantitative or qualitative composition 
either of solids or fluids. It may constitute so powerful a 
predhponent to disease as speedily to cause impairment of 
health, or it may be so slight, that without the co-operation 
xxvi. 69 
