CAUSES OF DISEASE. 25 
change or atrophy, as in old age. Want of blood, of nervous energy, or of use, 
are the common causes. Degeneration may be said to have occurred in an 
organ or a part of the body when there has been a change in its substance, 
Thus fat, by taking the place of the muscular tissue, may cause the heart to 
degenerate. Or the arteries may become degenerated in consequence of the 
intrusion of a bone-like material and the displacement by it of their proper sub- 
stance. In degeneration also there may be a hardening or softening of the 
parts affected. 
Morbid growths include warts, wens, bony enlargements, tumors, etc. The 
use of the term is generally restricted to growths which do not endanger life, 
and which are also called innocent; while others, like cancers, are designated 
malignant. 
CAUSES OF DISEASE. 
A knowledge of the causes of disease is not only necessary as a basis of pre- 
vention, but in many instances it is of much importance in effecting recovery. 
If the disease-producing agencies are known, their disturbing influences can, in 
a great measure, be obviated. After giving rise to disease the cause frequently 
continues active, and before a cure can be effected its removal is imperative. 
The essential causes and precise nature of many morbid conditions have been 
discovered, but progress in that direction is slow, and the field yet to be covered 
by investigators is wide. A growing understanding and an accumulation of facts 
and individual experiences, are materials from which, at some future period, will 
be developed truer perception and a more accurate knowledge of the causal con- 
nections of disease, and their conformity with fixed laws. 
As generally defined, causes are internal or external. The former are devel- 
oped within the animal economy, and are the results of impairments of certain 
vital processes. For instance, when the kidneys act imperfectly, urea, a poison- 
ous constituent of the urine, is retained in the blood and disease excited. Among 
the external causes are infectious and contagious matters, the different poisons, 
also wounds, injuries, and all harmful influences which exist independent of 
the animal organism yet are active agencies in inducing disease. 
Causes have also been distinguished as ordinary, and special or specific. Or- 
dinary causes are those to which all dogs are more or less exposed, each giving 
rise to certain forms of disease. They may also be termed unavoidable, and in- 
clude heredity, age, and sex. Special or specific causes are those which are only 
now and then encountered; and each cause gives rise invariably to but one form 
of disease. There is no satisfactory evidence that any of these causes ever arise 
spontaneously; in every case the pre-existence of the specific poison or organism 
is necessary. Tetanus and, beyond doubt, distemper, illustrate the diseases which 
