PHYSIOLOGY OF THE BLOOD. 395 
blood, either artificially or by natural haemorrhage. The next 
changes are excess or deficiency of its component parts. 
The blood frequently contains foreign matter, as pus, the 
contagious effluvia arising from diseases, gases, &c. Two fami- 
liar instances I will mention : in jaundice the bile enters the 
circulation and is distributed to every organized part of the body. 
In case of suppression of urine, urea may be detected in the 
blood. Prevost and Dumas, having secured the renal arteries of 
a dog, and excised the kidneys, succeeded in obtaining, two 
days after the operation, twenty grains of urea from five ounces 
of blood. The experiments of Christiorn and Dr. Prout sub- 
stantiated the correctness of this statement. Although the 
secretions cannot easily be detected in the blood, yet 1 am of 
opinion, could it be collected in sufficient quantity from the 
vessels immediately supplying the part, it is possible to be done; 
for instance, the calculi found in the kidneys and bladder, are 
not formed at once, but by the constant supply of blood, and, 
consequently, the gradual accumulation of the ingredients which 
compose them. % 
The blood may become morbid from the influence of impure 
air, also from a deficient quantity of food, or from noxious 
matter incorporated with the food. There are many interesting 
cases on record as proofs that deleterious substances may be 
conveyed, more or less promptly, into the circulating system 
through the means of diet. Marshall Hall observes, “ there is 
no point in pathology more instructive than that of scorbutus 
in the human subject. The morbid influence of salt meat, the 
curative influence of acids, the baneful effects of impure air 
and diet, and the beneficial effect of a change of atmosphere 
and regimen, are shewn more clearly in this disease than in any 
other.” He also remarks, ‘ ‘ that inflammation of serous mem- 
branes has chiefly the effect of producing the buffy appear- 
ance of blood when abstracted in such cases ; but inflammation 
of mucous membranes is seldom attended with this appearance, 
or frequently occurs without it.” I think this appearance de- 
ceptive, and, if depended on, without strict attention, calculated 
to mislead, as it is greatly influenced by circumstances : much 
depends on the size of the orifice from which you bleed. In 
cases of extreme debility I have seen the buffy appearance more 
developed than in any other, although not so firm or tough. 
Before I mention the changes the blood undergoes during 
respiration and circulation, I will briefly notice 
The Physiology of the Circulation, 
How many and important are the uses of this faithfully main- 
