24
2. TISSUE EXPOSURE
preparation for a perfusion, with particular care to avoid bubbles in the
line from the reservoir bottle.
Perfusion can be done most easily and effectively if it is limited to one
end or the other of an animal at any given time. To perfuse the head
one can work through the left ventricle most easily in the case of a small
animal such as a rat or mouse. But with a large animal such as a cat it
seems easier to us to use a retrograde perfusion, back along the thoracic
aorta, so that the cannula then can be tied in place securely. In the
latter case, the root of the aorta is clamped just before perfusion begins.
In either case, the descending aorta is clamped in the thorax before it
enters the abdominal cavity.
Preparation for placing the cannula includes opening the chest as
rapidly as possible as the animal dies. If the aorta is to be cannulated,
the ribs of the left side should be cut close to the backbone to allow a
very wide exposure. Some bleeding through intercostal arteries will occur
but only a few of the larger vessels ordinarily will require a hemostat. It
takes only seconds after the aorta has been exposed to push a tie under
it with curved blunt forceps, and then to cannulate it. Perfusion can be
started before the heart becomes quiescent. If the lower end of a large
animal is to be perfused, we prefer cannulating the aorta in a normal
direction, clamping it above the point of cannulation just before doing
this.
When the vascular system is to be used to distribute fixative it is important to realize that a large excess of fixative must be pumped through
rapidly. That is to say, a drainage route must be opened up so that
flushing can be vigorous at first. Thus, just before or just after perfusion
is begun, the vena cava in the vicinity of the heart, or the right atrium
should be clipped so that venous blood pours out in an unimpeded
fashion. Perfusion is begun suddenly as the stopcock or clamp just preceding the cannula is opened widely. There will be no blood clotting
which will impair the perfusion if there are no delays, and the perfusion
is allowed to proceed continuously with a uniform hydrostatic pressure.
Trouble is to be expected when there is some sort of an interruption, and
then an effort is made to reestablish perfusion. Even brief stasis is apt to
allow irreversible clotting to occur so that perfusion channels are permanently closed off, and at best an incomplete perfusion follows. Thus
there must be no hesitation once perfusion has been started. It must be
noted that there is no necessity for flushing out the blood with salt
solution before introducing an aldehyde fixative. This presumably would
2. TISSUE EXPOSURE
preparation for a perfusion, with particular care to avoid bubbles in the
line from the reservoir bottle.
Perfusion can be done most easily and effectively if it is limited to one
end or the other of an animal at any given time. To perfuse the head
one can work through the left ventricle most easily in the case of a small
animal such as a rat or mouse. But with a large animal such as a cat it
seems easier to us to use a retrograde perfusion, back along the thoracic
aorta, so that the cannula then can be tied in place securely. In the
latter case, the root of the aorta is clamped just before perfusion begins.
In either case, the descending aorta is clamped in the thorax before it
enters the abdominal cavity.
Preparation for placing the cannula includes opening the chest as
rapidly as possible as the animal dies. If the aorta is to be cannulated,
the ribs of the left side should be cut close to the backbone to allow a
very wide exposure. Some bleeding through intercostal arteries will occur
but only a few of the larger vessels ordinarily will require a hemostat. It
takes only seconds after the aorta has been exposed to push a tie under
it with curved blunt forceps, and then to cannulate it. Perfusion can be
started before the heart becomes quiescent. If the lower end of a large
animal is to be perfused, we prefer cannulating the aorta in a normal
direction, clamping it above the point of cannulation just before doing
this.
When the vascular system is to be used to distribute fixative it is important to realize that a large excess of fixative must be pumped through
rapidly. That is to say, a drainage route must be opened up so that
flushing can be vigorous at first. Thus, just before or just after perfusion
is begun, the vena cava in the vicinity of the heart, or the right atrium
should be clipped so that venous blood pours out in an unimpeded
fashion. Perfusion is begun suddenly as the stopcock or clamp just preceding the cannula is opened widely. There will be no blood clotting
which will impair the perfusion if there are no delays, and the perfusion
is allowed to proceed continuously with a uniform hydrostatic pressure.
Trouble is to be expected when there is some sort of an interruption, and
then an effort is made to reestablish perfusion. Even brief stasis is apt to
allow irreversible clotting to occur so that perfusion channels are permanently closed off, and at best an incomplete perfusion follows. Thus
there must be no hesitation once perfusion has been started. It must be
noted that there is no necessity for flushing out the blood with salt
solution before introducing an aldehyde fixative. This presumably would