I'lIYSIOLOGY. 
sailed by again introducing an into the 
lungs. Hence arises the importance of 
inflating the lungs in instances of apparent 
death from drowning, &c. in order to excite 
the heart to action. The mode in which 
the arteries and veins contribute to the 
circulation, ^ill be understood from the 
account of those vessels in the article Ana- 
TOMY. 
The circulation of the blood is different 
in the foetus, in consequence of differences 
in the structure of the organs devoted to 
this function. See Foetus. 
The situation of the child in utero pre- 
cludes the access of atmospheric air to the 
lungs ; these organs are consequently small 
and collapsed, and the lesser circulation 
can hardly be said to take place in the foetal 
state. Although its circulation might, in 
this respect, be considered as more simple 
than that of the adult, this function be- 
comes considerably complicated by the 
connection with the placent®. A portion 
only of the child’s blood circulates through 
this part, and it is no doubt so altered or 
modified by this passage through the vessels 
of the placenta, as to become more fit for 
the growth and nourishment of the child. 
No such alteration or modification has, 
however, been actually demonstrated in the 
foetal blood. Physiologists have discovered 
no difference in this fluid in the various ves- 
sels of the foetus. It is of the same dark 
colour in the arteries and veins. The inter- 
ruption of the communication with the 
placenta, before respiration has commenced, 
is, however, suddenly fatal. Our ignorance 
of the functions of the placenta, and of the 
liver, which is of immense size in the foetus, 
as well as of the changes which the foetal 
blood undergoes in the complicated system 
of organs, which are connected with its 
circulation in this state of existence, leaves 
many parts of the subject in doubt and 
obscurity. 
Respiration. The exposure of tlie blood 
to the atmospheric air, by which the chyle, 
that has entered the circulating system from 
the thoracic duct, is converted into blood, 
and by which those changes are effected in 
the whole mass of circulating fluid, which 
are essentially necessary to the continuation 
of life, takes place in the lungs. 
The respiratory organ has been aptly 
compared to an empty bladder, placed in a 
pair of bellows, with its neck adapted to the 
instrument, and giving entrance to a ^co- 
lumn of air when the sides are separated. 
In breathing, the dilatation of tlie chest 
occasions the lungs to enlarge by the en- 
trance of air into tjiem from without ; these 
viscera not possessing any means of enlarge- 
ment in themselves; this is termed inspi- 
ration. The expulsion of the air, after it 
has served the purposes of respiration, by 
means of a process exactly contrary to the 
former, is called expiration. The diaphragm 
and the abdominal muscles are the chief 
agents in enlarging and diminishing the 
chest. The former muscle in its relaxed 
state is strongly arched, and the convexity 
of this arch is towards the chest. Its curved 
fibres become straight by the contraction : 
the whole muscle descends towards the ab- 
domen, and pushes the abdominal viscera, 
which lie against its under or concave sur- 
face, downwards and forwards. Hence 
the surface of the belly rises when we draw 
air into the chest. In the next moment, 
the abdominal muscles contract and push 
back the viscera, and thereby diminish 
the chest in a degree proportionate to its 
former enlargement. The increase of the ' 
thorax, effected in this way, takes place 
in the perpendicular direction ; but it may 
also be enlarged in its whole diameter 
by means of the intercostal muscles, which 
by elevating and twisting outwards all the 
ribs, push the sternum forwards, and en- 
large the chest in every direction. When 
the action of these powers has ceased, tlie 
natural elasticity of the parts restores the 
parietes of the thorax to their former posi- 
tion. In natural respiration both these me- 
thods of altering the capacity of the chest 
are employed ; but females seem to use the 
intercostal muscles more than the male sub- 
ject, as the heaving of their bosom demon- 
strates j yet breathing can be carried on by 
either method, to the exclusion of the 
other ; as we sometimes see under circum- 
stances of accident or disease. In the case 
of a broken rib, where the rubbing of the 
broken ends would be highly painful, the 
chest is bound up so as to render the ribs 
motionless, and the diaphragm and abdo- 
minal muscles perform the whole business 
of respiration. When the diaphragm and 
abdominal muscles act together, they com- 
press the viscera between them, and the 
pressure thus produced, assists in the ex- 
pulsion of their contents. This effort is 
termed straining, and is seen in vomiting, in 
the [act of discharging the feces and urine, 
and in the propulsion of the child from the 
uterus. 
The state of the mind considerably affects 
the mode of respiration, although Ae mus- 
