526 
HEREDITARY DISEASES OF HORSES. 
stitution, in order to illustrate more fully and satisfactorily 
the hereditary tendencies of disease, which we shall now pro- 
ceed to discuss. 
Hereditary diseases exhibit certain eminently character- 
istic phenomena, some of which we shall here enumerate : 
1. They are transmitted by the male as well as by the 
female parent, and are doubly severe in the offspring of 
parents both of which have been affected by them. 
2. They develop themselves, not only in the immediate 
progeny of animals affected by them, but also in many sub- 
sequent generations. 
3. They do not, however, always appear in each generation 
exactly in the same form. One disease is sometimes substi- 
tuted for another analogous to it, and this, after some genera- 
tions, becomes again changed into that to which the breed 
was originally liable. Thus, stocks of cattle previously sub- 
ject to phthisis often become affected for several generations 
with dysentery, to the total exclusion of phthisis, but by and 
by the dysentery disappears to give place to the phthisis. 
4. Hereditary diseases occur to a certain extent indepen- 
dently of external circumstances, appearing under all sorts of 
management, and being little affected by changes of locality, 
separation from the diseased stock, or such other causes as 
modify the production of non-hereditary diseases. 
5. They are, however, most certainly and speedily deve- 
loped in circumstances inimical to general good health, and 
often occur at certain so-called critical periods of life, when 
unusual demands on the vital powers take place. 
6. They show a striking tendency to modify and absorb 
into themselves all extraneous diseases. For example, in an 
animal of a consumptive constitution, pneumonia seldom runs 
its ordinary course, and, when arrested, often passes into con- 
sumption. 
7. Hereditary diseases are less effectually treated by ordi- 
nary remedies than other diseases. Thus, although an attack 
of phthisis, rheumatism, or constitutional ophthalmia, may 
be subdued, and the patient put out of pain and danger, the 
tendency to the disease will still remain, and be greatly ag- 
gravated by each attack. 
Hereditary diseases do not necessarily show themselves at 
birth. In horses and cattle there are only a few which do so. 
The scrofulous diathesis sometimes presents itself in large 
collections of pus, which occasionally prove fatal within a few 
days after birth ; and symptoms of hydrocephalus, rickets, 
and occasionally rheumatism — all hereditary complaints — are 
also sometimes found present at that early period. But most 
