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VARIATIONS IN THE BLOOD, AND IN ITS DISTRIBUTION. 677
received, a local hyperemia having been called forth. This simple
experiment will serve to illustrate a principle which seems of import-
ance in the study of the subject, and constitutes the basis of a division
of the local hyperemias into two classes. It has already been suffi-
ciently explained that simple hypersemia, or local dilatation of the vas-
cular svstem throusjh reflex action, never g-ives rise to inflammation. It
is evident, however, that in the experiment just related a continuance or
increased severity of the cause might beget the inflammatory process.
This is true of very many of the causes of local hypersemia, and many
of them are actually followed by inflammation ; therefore they may be
designated as hypercemian of irritation. In this case the inflammation
is not in consequence of the hypersemia, for we have seen that inflam-
mation does not necessarily follow the most extended dilatation of the
vessels in complete paralysis of the vaso-motor nerves : the inflamma-
tion is the result of tissue injury.
Increase of blood-pressure is not often local, and even if it were it
could not ordinarily j)roduce hyperemia. The vessels of the general
system cannot be dilated to any considerable degree by this cause. In
the lungs, however, the case is different. Here the object is the aeration
of the blood, and the vessels are so arranged in the walls of the air-
vesicles that the blood is spread out in very thin sheets. These capil-
lary sheets are not composed of simple round anastomosing ca])illary
twigs, as in the other parts of the body, but in the normal condition of
expansion of the lung they are distinctly flattened. This renders them
easily distensible by increase of pressure.^ Therefore we are liable to
active congestion of these organs in consequence of sudden increase of
the blood-pressure from any cause.
Passit^e hypercemia occurs whenever there is obstruction to the
flow of blood away from the j)art by the veins. In this case the cap-
illary system of the region becomes overfilled with blood, which, on
account of retention or unusually slow movement, becomes highly
venous in character. This is also called passive congeMion, venous
hypercemia , or congestion. This state is seen in connection with debil-
ity or enfeeblement of the heart. In the normal state of the circula-
tion the blood is urged forward with sufficient power to cause it to
ascend from dependent parts against gravity without perceptible hin-
drance ; but if the heart, which is the principal motive-power of tlie
circulation, becomes weakened or disabled in its valves, the blood fails
to return promptly by the veins, and stagnation is the result. The
effect is the same if the veins be so obstructed that there is only a
partial return of the blood from a part, the circulation being otherwise
good. The congestion, except in the lungs, is probably in no case
materially increased by arterial pressure, as was formerly supposed.
TJie dilatation of the capillaries is a reflex phenomenon occurring in
response to the needs of the tissues for arterial blood. Every individual
living cell in the organism must have access to free oxygen, must absorb
oxygen and exhale carbonic acid must breathe in order to maintain its
' See monograph, "The Circulation of the Blood in the Air-vesicles of the Lungs,"
bv G. V. Black, D. D. S., St. Louis Med. and Surg. Journal, and 3Iissouri Dental Journal^
1878.
VARIATIONS IN THE BLOOD, AND IN ITS DISTRIBUTION. 677
received, a local hyperemia having been called forth. This simple
experiment will serve to illustrate a principle which seems of import-
ance in the study of the subject, and constitutes the basis of a division
of the local hyperemias into two classes. It has already been suffi-
ciently explained that simple hypersemia, or local dilatation of the vas-
cular svstem throusjh reflex action, never g-ives rise to inflammation. It
is evident, however, that in the experiment just related a continuance or
increased severity of the cause might beget the inflammatory process.
This is true of very many of the causes of local hypersemia, and many
of them are actually followed by inflammation ; therefore they may be
designated as hypercemian of irritation. In this case the inflammation
is not in consequence of the hypersemia, for we have seen that inflam-
mation does not necessarily follow the most extended dilatation of the
vessels in complete paralysis of the vaso-motor nerves : the inflamma-
tion is the result of tissue injury.
Increase of blood-pressure is not often local, and even if it were it
could not ordinarily j)roduce hyperemia. The vessels of the general
system cannot be dilated to any considerable degree by this cause. In
the lungs, however, the case is different. Here the object is the aeration
of the blood, and the vessels are so arranged in the walls of the air-
vesicles that the blood is spread out in very thin sheets. These capil-
lary sheets are not composed of simple round anastomosing ca])illary
twigs, as in the other parts of the body, but in the normal condition of
expansion of the lung they are distinctly flattened. This renders them
easily distensible by increase of pressure.^ Therefore we are liable to
active congestion of these organs in consequence of sudden increase of
the blood-pressure from any cause.
Passit^e hypercemia occurs whenever there is obstruction to the
flow of blood away from the j)art by the veins. In this case the cap-
illary system of the region becomes overfilled with blood, which, on
account of retention or unusually slow movement, becomes highly
venous in character. This is also called passive congeMion, venous
hypercemia , or congestion. This state is seen in connection with debil-
ity or enfeeblement of the heart. In the normal state of the circula-
tion the blood is urged forward with sufficient power to cause it to
ascend from dependent parts against gravity without perceptible hin-
drance ; but if the heart, which is the principal motive-power of tlie
circulation, becomes weakened or disabled in its valves, the blood fails
to return promptly by the veins, and stagnation is the result. The
effect is the same if the veins be so obstructed that there is only a
partial return of the blood from a part, the circulation being otherwise
good. The congestion, except in the lungs, is probably in no case
materially increased by arterial pressure, as was formerly supposed.
TJie dilatation of the capillaries is a reflex phenomenon occurring in
response to the needs of the tissues for arterial blood. Every individual
living cell in the organism must have access to free oxygen, must absorb
oxygen and exhale carbonic acid must breathe in order to maintain its
' See monograph, "The Circulation of the Blood in the Air-vesicles of the Lungs,"
bv G. V. Black, D. D. S., St. Louis Med. and Surg. Journal, and 3Iissouri Dental Journal^
1878.
