2) 
NATURE 
[ DECEMBER 31, 1896 
and are therefore, probably, of little importance so far as we are 
concerned. Keeping this in view, I have always incubated my 
plates coated with agar-agar at the body temperature, in order 
to gain information as to the approximate number of organisms 
which are likely to find access to our respiratory tract, and 
which have a chance of thriving there. I have not attempted 
to separate the aerobic organisms from the anaerobic ones, 7.e. 
those which can grow in the presence of oxygen from those 
which cannot. All I wish to show is that under ordinary con- 
ditions of life we must breathe an air which contains bacteria, 
sometimes many bacteria. These plates do not tell us how 
many bacteria we inhale in a given time; they simply tell us 
that the air which we breathe is not sterile. 7 
The bacterial flora of air varies considerably. The lady shop- 
ping in Oxford Street will inhale more bacteria than the boy 
who runs about on Wandsworth Common. 
We all expect to find that the air in a railway carriage of the 
Underground Railway is full of bacteria ; but, although very 
rich in bacteria, it is not so impure as might have been 
anticipated. 
I have prepared a number of plates from the air in my | 
laboratory, exposing them from one to five minutes. Some of 
them are very full of colonies, others less ; and this depends on the 
number of students that have been at work during that time. 
The more students the more dust, and therefore the more 
bacteria. 
Wherever many people are congregated the air becomes laden 
with dust and bacteria. Thus plates exposed in the Surgery of 
one of our largest general hospitals for three to five minutes are 
covered with numerous colonies. Compare this with the air in 
the quadrangle of the same hospital, and you will see the effect of 
confinement and of crowding together. In the former case we 
find numerous colonies, while in the latter case—z.e. in the 
quadrangle—the air is much freer from micro-organisms. This 
is also shown by plates taken from the Apothecary’s Shop of 
the same hospital during a time when the patients collect to 
obtain their medicines ; here the air is laden with bacteria. 
If you desire a further example, you will find it in plates 
prepared in the Smithfield meat-market. After a minute's 
exposure already they are covered with colonies ; and we cannot 
wonder at this, if we remember how active the life there is, and 
how much organic matter is carried about. 
. In foggy or misty weather, when the air is quiet, the number 
of organisms is greatly reduced. It requires, therefore, but 
little reflection to recognise that, under ordinary conditions, the 
air which we breathe contains numerous bacteria: we live in 
a world which is not sterile, and, therefore, unless there exist 
special preventive measures, those body cavities which are in 
direct communication with the outer world must also contain 
bacteria. The mouth, the alimentary and respiratory tracts, 
and the pores of our skin are all in direct communication 
with space outside us, in fact, from the bacteriological point of 
view they represent simply the outer world. 
We may, therefore, expect that the organisms which exist out- 
side, in part at least, also find their way into these body spaces or 
cavities, even if they were not carried into the mouth with our 
food, or into the nose by the process of respiration. We cannot 
possibly prevent the bacteria from entering the mouth, even if 
we refused any but sterilised food. This is an important point 
to remember, because it proves the impossibility of excluding 
bacteria from the digestive tract. Saliva always contains them, 
often in great numbers ; and as saliva is constantly swallowed, 
they must find their way into the stomach. But to return to 
‘the bacteria which we breathe. The air passages, z.e, the nose, 
larynx, trachea, bronchi, and their ramifications, and the alveoli 
or air spaces of the lungs contain bacteria. In normal respira- 
tion the inspired air enters the nose, but the anatomical structure 
of the latter is such as to act as a bacterial filter, imperfect 
no doubt, but still capable of retaining from three-fourths to 
four-fifths of the bacteria of the inspired air, Therefore, 
although large numbers of bacteria find their way into the 
nasal cavities, the true mucous membrane of the nose is, sur- 
prisingly poor in bacteria ; and this to some extent is due to 
filtration, and to the fact that healthy nasal mucus possesses 
considerable bactericidal or disinfecting power. If, however, 
the nasal mucous membrane is diseased, and it is frequently 
dliseased in this country, large numbers of organisms may be 
found. The nose, therefore, is an important bacterial filter, 
and it follows that breathing through the nose is the best 
method which the body possesses for the purification of the air 
NO. 1418, VOL. 55] 
r 
which we breathe. When we are forced to breathe through 
the mouth, bacteria are readily inhaled into the larynx. The 
latter, as well as the trachea, bronchi, and lungs in man 
always contain bacteria, because the nose is a very imperfect 
filter which often gets out of order, and also because the 
respiratory tract is in direct communication with the outer world. 
It is stated that the trachea, bronchi, and lungs of animals 
(rabbits) are almost free from bacteria ; it is certainly not so in 
man. 
The air which we breathe contains both organisms which are 
capable of producing disease, and organisms which are harmless. 
The latter are far more numerous ; still pathogenic organisms, 
z.é. disease-producing organisms, do float about in the air, and 
may then be inhaled. The organisms which we find in the nose, 
mouth, larynx, and lung include some undoubtedly pathogenic 
forms, as for instance the micro-organisms of pneumonia, sup- 
puration, &c. These exist in space around us, and therefore, 
unless they are destroyed in the respiratory passages, they must 
find their way into the cavities of the body which are in direct 
communication with the outer world; and thus we see that 
virulent organisms may enter the body and remain there without 
causing any lesions, for although we frequently inhale pathogenic 
organisms we do not inhale the diseases which they are capable 
of producing. The bacteria enter the body, but not its tissues ; 
they thrive in the secretions and on the mucous membranes 
lining the various cavities of the body. But that only means 
that they are practically still outside the body proper. It is a 
common error to say that because an organism is found inside 
some space or cavity of the human body, that therefore it lives 
in the tissues or in the body. As I said before, all the cavities 
and spaces in direct communication with the outer world are the 
outer world, and 
we may expect in them the same 
organisms as occur in the outer world. The resist- 
ance of our tissues in health, and the absence of 
predisposing influences prevent the pathogenic organisms 
present from doing more than leading a harmless or 
saprophytic existence ; but if for some reason or another they 
actually enter or irritate the tissues, the most serious forms of 
disease may appear, as for instance pneumonia. Many: of us 
carry the organism of this disease about in our mouths, bronchioles 
or alveoli, although we remain perfectly healthy ; pneumonia, 
however, frequently appears after a drenching or a chill. The 
coccus of pneumonia, which lay harmlessly on the mucous 
membrane, now assumes a virulent character, invades the lung 
tissues, and in some cases even the circulation. The bacillus of 
tuberculosis in rare cases has also been found in the nasal 
mucous membrane of individuals attending on consumptives ; it 
did, however, no harm so long as it was outside the tissues on 
the mucous membrane, #.e. in the outer world whence it had 
come. 
Anyhow, we must recognise that since, under ordinary con- 
ditions, we live amongst bacteria and decomposing matter, 
we must be inhaling large numbers of bacteria into our nose, 
mouth, larynx, trachea, bronchiand lungs ; and that since patho- 
genic bacteria from time to time occur in dust, these also must find 
their way into those spaces and tracts. But we need not feel 
alarmed and insist on a sterile supply of air, because the danger 
of aerial infection is but slight, and because the survivors 
amongst the inhaled micro-organisms will remain harmless, 
unless the system is weakened or rudely disturbed by some 
interference. It is, however, well to remember that our respira- 
tory passages may, and generally, perhaps, do contain numerous 
germs capable of producing disease and death, and that these 
germs may lie dormant there for a long time, ready under pro- 
vocation to do their worst. 
There is one other organism I wish to single out, because I 
shall have much to say about it subsequently, that is the Bac- 
terium colé commune. This organism I have always found in 
saliva and in sputum, on the tonsils and on the pharynx. It is 
an ubiquitous organism outside the human. body, and therefore 
occurs in the body spaces and, especially, in the intestines which 
are continuous with the outer world ; and it would be surprising 
if it did not. : 
We may here conveniently consider -the flora of the mouth 
and pharynx. Whatever micro-organisms are present in the 
mouth must have got there from the air or the food. At a 
particular moment there may be an enormous number of 
organisms present, but many of them are merely temporary 
visitors ; they either die because they do not find suitable condi- 
tions, or they are passed on into the stomach and intestines. 
