stages of tuberculosis, because, we may assume, they experience little 
pain or distress. Cattle, with their lower perception and compar- 
atively insignificant means to express suffering, do not complain at 
all because of the pain and distress tuberculosis causes them. 
The frequency with which tuberculous subjects cough depends 
largely upon perception, or sensations in their lungs and throats 
that prompt them to cough. Cattle, shown on autopsy to have ex- 
tensive, advanced, tuberculous lesions of the lung, though observed 
long periods of time before their death, were found to cough only a 
little more frequently than cattle shown on autopsy to have healthy 
lungs. When tuberculous cattle cough it is usually a single, accel- 
erated expiratory effort, or at most two or three such efforts in suc- 
cession, which is sufficient to raise the material that has accumulated 
in their larger air passages far enough into their mouths to be swal- 
lowed. Expectoration, which is common with persons, does not oc- 
cur, and paroxysms of sustained coughing, also common with per- 
sons, are very rare and occur only during the last stages of pulmonary 
tuberculosis. 
The bodies of all animals are stronger and more capable, as a whole 
as well as in their individual parts, than the ordinary vicissitudes of 
life require them to be. The difference between the actual strength 
and capability and that ordinarily required is known as the factor 
of safety. The rarely interrupted, placid routine of a cow’s life 
enables her to derive full benefit from the factors of safety in her 
body when she becomes affected with a slow, chronic disease like tuber- 
culosisj the lesions of which are circumscribed in the sense that they 
do not seriously affect parts of the body remote to those in which 
they are located; hence the factors of safety greatly help to prevent 
tuberculous disease in the bodies of cows from showing itself by 
external symptoms. An example of this is seen in illustration Xo. 15, 
which shows a cow affected with advanced tuberculosis. 
The tuberculous disease of the cow represented in the picture is 
partly located in the glands of her throat : the enlarged glands press 
on and narrow the passage through which air reaches her lung to 
such an extent that she almost dies of suffocation when she is driven 
a short distance at a moderately fast walk. The general condition 
of the cow shows that the opening through which the air she breathes 
must pass is still large enough for ordinary purposes; the difference 
between its original and its present size is a factor of safety that has 
been almost wholly lost. 
We may conclude that, with few exceptions, the character of tuber- 
culosis among cattle is that of an effectually concealed disease, the 
detection of which before it is well advanced and has done great 
harm is practically impossible through the agency of our unaided 
powers of observation. 
