ON THE EXTERNAL CAUSES OF DISEASE. 
263 
Mr. Vines, the demonstrator of the Veterinary College, has 
explained this in a much abler manner. When an animal is 
in possession of the highest possible degree of health and 
strength, and performing moderate exertion, the muscles are of a 
bright red colour ; and if blood be drawn, it will quickly coagu¬ 
late into a solid red mass, and give out a small quantity of serum ; 
whilst on the contrary, if the same animal be exposed to severe 
cold, and supplied only with little food, the blood will be found 
to coagulate much more slowly, and to separate into two parts— , 
white and dark blood, constituting what is called buffy or in¬ 
flamed blood ; and in proportion to the intensity of the cold, and 
deprivation of food, so will the blood assume the buffy ap¬ 
pearance.” 
Thus the different colours of the blood indicate its various de¬ 
grees of vitality; for, in animals possessing health and strength, 
it appears red from possessing a high degree of vitality; while 
in those where the systems are unhealthy and debilitated, it is 
white, from having a less degree of vital power. 
Here, also, we shall find, on investigation, a fruitful source for 
the causes of many diseases, according to the changes wdiich the 
blood may undergo as to its composition. 
This leads us to the second part of our subject, thatj in all 
diseases arising from atrial poison, the blood is contaminated.^' . 
It is a well known fact, that putrid animal and vegetable mat¬ 
ter is capable of communicating to the air a poisonous quality, 
which gives a tendency to living bodies to assume certain forms 
of disease. In all those diseases the blood is more or less empoi¬ 
soned. This empoisoning may take place in two different ways. 
The aerial poison may either be imbibed by the absorbent ves¬ 
sels on the surface of the skin, or it may-be attracted immediately 
into the circulatory current, along with the atmospheric air in 
the lungs. In either instance it must sooner or later enter into 
the circulation, and derange the physical properties of the blood. 
And in both instances, the air is the medium through which the 
poison is communicated ; as it has been repeatedly proved, parti¬ 
cularly by Dr. Barry, with a variety of poisons, on the surface 
of the skins of several species of animals, .that, when the pressure 
