Anesthesia is divided into three stages; the stimulant, anaesthetic and
paralytic.
_In the first stage_ there is struggling and excitement, due partly to
the action of the drug and partly to fright. The local irritant action
of the vapor causes choking and coughing, which also induces struggling.
The respiratory and cardiac centers are temporarily stimulated, as a
consequence of which the pulse and respiratory movements are increased
in force and frequency and blood tension is raised.
The smaller animals, particularly the dog, may vomit during the first
stage of anaesthesia. In the first stage the dog may bark, whine or
howl, the horse neighs and groans; other animals give expression to
sounds more or less characteristic to their species.
_The second or anesthetic stage_ is characterized by loss of
consciousness, sensation, motion and partial loss of reflex action and
is that state suitable for operations. The stimulating action of the
anesthetic has passed and there is now depression of the cerebral
functions, the motor centers. The voluntary muscles are completely
relaxed, the sphincters occasionally, the patient lies absolutely
motionless, the cornea fails to respond to irritation, i. e., winking is
not produced when the cornea is lightly touched with the finger.
Sometimes the muscles are rigid and twitching during this stage of
anesthesia, though sensation and consciousness are absent. In the
anesthesia stage the pulse is slow, full and strong, due to lowered
blood pressure, the breathing is slow and shallow but regular.
_The third or paralytic stage_, which must be carefully watched against,
poisoning is beginning and there is depression of the three great
medullary centers controlling the heart, respiration and vascular
tension and also the posterior reflex centers of the spinal cord, so
that the urine and faeces are passed involuntarily. The passage of urine
frequently occurs in the first stages of anesthesia and should not of
itself be considered a danger mark. When the pulse becomes rapid, feeble
and irregular, the breathing is at first stertorous and then the
respiratory movements become shallow and weak, with long intervals
intervening between them; this irregularity is a most important danger
sign. The skin and mucous membrane often become cold and clammy. The
pupils are usually widely dilated, though death may occur with either
dilated or contracted pupils and consequently no dependence should be
put in this sign unless there has been a sudden change from one
condition of the pupils to the other. The three above mentioned stages
are conventional, and are not in any case so clearly defined in practice
as they are described theoretically upon paper. The first stage may be
either absent or prolonged, and the last stage should not be reached at
all.