THE RESPIRATORY SYSTEM. AN1 
It is customary to distinguish tidal, complementary, supple- 
mentary, and residual air. 
The vital capacity is estimated by the quantity of air the 
respiratory organs can move, and is very variable. 
The blood is the respiratory tissue, through the mediation 
of its red cells, by the hemoglobin they contain. This sub- 
stance is a ferruginous proteid, capable of crystallization, and 
assuming under chemical treatment many modifications. When 
it contains all the oxygen it can retain, it is said to be saturated, 
and is called oxy-hemoglobin, in which form it exists (with 
some reduced hemoglobin) in arterial ‘blood, and to a lesser 
extent in venous blood, which differs from arterial in the rela- 
tive proportions of hemoglobin (reduced) it contains, as viewed 
from the respiratory standpoint. 
Oxy-hemoglobin does not assume or part with its oxygen, 
according to the Henry-Dalton law of pressures, nor is this gas 
in a state of ordinary chemical combination. It is found that 
the oxygen tension of the blood is lower and that of carbonic 
anhydride higher than in the air of the alveoli of the lungs, 
while the same may be said of the tissues and the blood re- 
spectively. This has been, however, recently again denied. 
Respiration is a vital process, though certain physical con- 
ditions (temperature and pressure) must be rigidly maintained 
in order that the gaseous interchanges shall take place. Res- 
piration is always fundamentally bound up with the metabo- 
lism of the tissues themselves. All animal cells, whether they 
exist as unicellular animals (Amoeba) or as the components of 
complex organs, use up oxygen and produce carbonic dioxide. 
Respiratory organs, usually so called, and the respiratory tissue 
par excellence (the blood) are only supplementary mechanisms 
to facilitate tissue respiration. Carbonic anhydride exists in 
blood probably in combination with sodium salts, though the 
whole matter is very obscure. 
Respiration, like all the other functions of the body, is con- 
trolled by the central nervous system through nerves. The 
medulla oblongata is chiefly concerned, and especially one 
small part of it known as the respiratory center. It is possi- 
ble, even probable, that there are subordinate centers in the 
cord, which, under peculiar circumstances, assume importance; 
but how far they act in concert with the medullary center, or 
whether they act at all when normal conditions prevail, is an 
open question. 
The vagus is the principal afferent respiratory nerve. The 
