Anaesthetics are remedies which destroy sensation.

It has already been mentioned that both sensation and pain require for their perception a certain condition of the cerebral centres and of the sensory nerves and spinal cord, by which impressions are conveyed to these centres.

The difference between anaesthetics and anodynes is to a great extent one of degree. Anodynes affect more particularly the cerebral centres by which pain is perceived, or the conducting paths by which painful impressions are transmitted, and thus in moderate doses lessen pain without destroying reflex action. They only affect the ordinary centres for reflex action when the dose is considerably increased. Anaesthetics, on the other hand, affect the cerebral and spinal centres more equally, and so abolish pain, ordinary sensation, and reflex excitability more nearly at the same time, though their abolition is by no means completely simultaneous.

According to Eulenberg, in chloroform-narcosis the patellar reflex is abolished first, then reflex from the skin, then from the conjunctiva, and lastly from the nose. As the anaesthesia passes off they return in the inverse order, patellar reflex being the last to reappear. A stage of excitement generally precedes the disappearance of patellar reflex, both in man and animals.

Narcosis by ether differs from that of chloroform in the much greater increase of patellar and other tendon reflexes, both in extent and duration.

Chloral hydrate and potassium bromide have an action like chloroform, but much weaker. Like chloroform, they paralyse the patellar reflex before the corneal reflex, but butyl-chloral (croton-chloral) paralyses the corneal reflex before the patellar.

In ordinary sleep, reflexes disappear in the same order as in chloroform narcosis, but in mesmeric sleep the reflexes are increased as in narcosis from ether. In hysterical conditions diminution of the cerebral reflexes from the nose and cornea with persistence of the patellar reflex has been observed.

The reflex power of the vaso-motor centre is very quickly paralysed by chloroform, so that irritation of a sensory nerve will no longer raise the blood-pressure. Its reflex power is much less affected by ether.1

Anaesthetics may be divided into local and general. The local are those which abolish the sensibility of the peripheral nerves of a particular area. The general are those which act on the central nervous system in the way already described, and abolish sensation throughout the whole body.

The chief local anaesthetics are cold, cocaine, carbolic acid, iodoform.

For the purpose of producing local anaesthesia, cold is generally applied by means of ether spray, until the part is all but frozen and is insensible, when slight operations may be made without the patient feeling any pain. The ether may perhaps have itself a certain amount of physiological effect in diminishing sensibility when applied in this manner. Carbolic acid painted over the surface also causes it to become white and to lose its sensibility, and may thus be used to lessen the pain of opening an abscess.

1 H. P. Bowditch and C. S. Minot, Boston Med. and Surg. Journ., May 21, 1874.

General anaesthetics are Nitrous oxide. Ether. Chloroform., Bromoform. Tetrachloride of carbon.

Trichlorhydrin. Bi-chloride of methylene. Paraldehyde. Bi-chloride of ethidene. Bromide of ethyl.

With the exception of nitrous oxide they all belong to the class of alcohols and ethers, and the substitution-compounds having an anaesthetic action are probably almost indefinite in number. Even alcohol itself produces general anaesthesia when volatilised and inhaled.

General Anaesthetics may destroy the sensibility of the nerve-centres indirectly or directly. Anaesthesia is induced indirectly by stopping the circulation in the brain and thus arresting the process of oxidation and tissue-change in the nerve-cells which are necessary for their functional activity.

This result may be produced by draining the blood from the head into other parts of the body. Thus in some of the hospitals at Paris, before anaesthetics were introduced, a plan was sometimes employed of rendering a patient insensible before an operation, by laying him flat on the ground, and then lifting him very suddenly to a standing posture by the united efforts of six or eight men (cf. pp. 193, 198).

Local arrest of the circulation to the brain by ligatures or by compression of the arteries has a similar effect. Waller has recommended diminution of the cerebral circulation, by the combined effects of simultaneous pressure on the carotid arteries and vagus nerves, as an easy means of producing anaesthesia for short operations.

Slight anaesthesia, usually accompanied by some giddiness, may be produced by taking a number of deep breaths in rapid succession. This may be used in order to lessen the irritability of the pharynx in laryngoscopic examinations, and to lessen the pain of opening boils or abscesses. The anaesthesia thus produced may perhaps depend on anaemia of the brain, although this is not certain.

Anaesthesia may also be produced by diminishing the internal respiration of the nerve-cells through a gradually increasing venous condition of the blood. Thus gradual suffocation by charcoal fumes or carbon monoxide causes complete insensibility, and the inhalation of nitrogen and of nitrous oxide has a similar action.

Anaesthesia may be caused by the direct action of drugs on the nerve-cells themselves. Chloroform, ether, and other allied substances belonging to the alcohol series appear to act in this way. Although their action is generally exerted through the blood by which they are conveyed to the brain when inhaled, yet they will also produce a similar action if locally applied to the nerve-centres. Thus Prevostl found that chloroform applied directly to the brain of a frog narcotises it when the aorta is tied. When the aorta is again unligatured, so that the current of blood can again wash the chloroform away, the narcosis disappears. Chloroform and ether when inhaled appear to act like alcohol, producing paralysis of the nerve-centres, commencing with the highest and proceeding downwards. The rate of paralysis, though the same in order, is more rapid than that caused by alcohol.

These anaesthetics are, however, not nerve-poisons only; they are protoplasmic poisons affecting simple organisms, such as amoebae and leucocytes, and destroying also the irritability of muscular fibre.

This action of anaesthetics and especially that of chloroform upon muscular fibre is one of considerable importance in reference to the occasional stoppage of the heart and consequent death during the administration of anaesthetics.

The action of anaesthetics may be divided into four stages: 1st. The stimulant stage.

2nd. The narcotic and anodyne stage.

3rd. Anaesthetic stage.

4th. Paralytic stage.

Stimulant Stage

Chloroform and ether, as already mentioned, resemble alcohol in their action, and, like it, in small doses will produce a condition of stimulation and acceleration of the circulation passing gradually into one of narcosis, in which the action of the higher nervous centres is more or less abolished, while that of the lower centres still remains.

In small quantities chloroform and ether are sometimes taken, either internally or by inhalation, for their stimulant effect. They are useful in lessening pain and spasm, as in neuralgia, and biliary, renal, or intestinal colic, when given till the stimulant is just passing into the narcotic stage.

Narcotic Stage

When pushed still further, sensibility becomes more impaired, reflex action still continues, and sometimes, just as in drunkenness, there is a form of wild delirium and great excitement. This is much less marked in feeble or debilitated persons than in strong men. In the latter, the struggles which occur in this condition are sometimes exceedingly violent, the patient raising himself forcibly from the couch, his muscles being in a state of violent contraction, the face livid, the veins turgid, and eyeballs protruding. Usually this condition quickly subsides and passes into the third stage - that of complete anaesthesia.

1 Prevost Practitioner July 1881.

In order to lessen the pains of labour, anaesthesia is usually carried to the commencement of the second stage.

Anaesthetic Stage

The third stage differs from the second, in the function of the spinal cord being abolished, as well as those of the brain; ordinary reflex is consequently abolished, and the most common way of ascertaining whether this stage has set in or not is by drawing up the eyelid and touching the conjunctiva. If no reflex contraction of the eyelid occurs, the anaesthesia is complete. By careful and judicious administration of the anaesthetic this condition may be kept up for a length of time even for hours, or days; but if the inhalation be carried too far, the anaesthetic passes into the fourth stage.

The third stage is the one employed for surgical operations.

Paralytic Stage

In the fourth the respiratory centre becomes paralysed, respiration ceases, and the beats of the heart become feebler and may cease altogether.

Uses of Anaesthetics.

Anaesthetics are used not only to lessen pain but to relax muscular action and spasm. They are chiefly employed to lessen pain in surgical operations, in labour, and in biliary and renal colic. They are used to lessen muscular action and spasm in tetanus, in poisoning by strychnine, in hydrophobia, and in the reduction of dislocations, fractures, and hernia. They are also of assistance in diagnosis, by allowing careful examination to be made of parts which are too tender or painful to be examined without it, and by causing the phantom tumours due to spasmodic contraction of the muscles to disappear.