This section is from the book "Materia Medica And Therapeutics: An Introduction to the National Treatment of Disease", by John Mitchell Bruce. Also available from Amazon: The pharmacology and therapeutics of the materia medica.
Morphia enters the circulation less quickly than some other alkaloids, although the first traces of the drug are rapidly discovered in the blood. Thus its full action is comparatively slowly developed, and solid opium continues to exert local effects even in the colon, portion by portion of the morphia being absorbed into the vessels. The red corpuscles are said to be reduced in size indirectly, possibly through slowing of the circulation and want of oxygen.
After adniinistration morphia may be found in all the organs, which, probably without exception, are physiologically affected by it; but its principal action is exerted upon the nervous system.
The convolutions are first briefly excited, and afterwards depressed, probably by direct action of the morphia upon the nerve-cells, not on the cerebral vessels. The stage of opium excitement transcends even the first stage of alcoholic intoxica-tion in the exaltation of feelings, the sense of happiness and comfort, the brilliancy of imagination, and the increase of in-tellectual power and mental vigour generally, all accompanied by brightness of expression and manner. But the effect of opium, even in this stage, is rarely one of pure exaltation, and in most persons is perhaps never so. There is generally some perversion of the faculties, and the imagination becomes extra-vacant, wandering into the land of dreams, of the grotesque, and the impossible. Depression now supervenes: the various perceptive and sensory centres in the convolutions are more or less depressed, according to the dose; impressions made upon the afferent nerves, including pain, do not readily affect the receptive centres; the subject becomes drowsy, and finally sleeps; and if he momentarily respond to a sharp enquiry or other forms of stimulation, he quickly relapses into heavy sopor. If the dose has been excessive, the stage of excitement is entirely absent, the cerebrum is speedily and profoundly de-pressed, and no response follows severe forms of stimulation, such as flgelliation: the patient is comatose. These effects of opium on the brain as a stimulant, hypnotic, anodyne, and narcotic, are more marked in man and in highly intellectual races than in animals and lower races respectively. In coldblooded animal they are quite subordinate to the effects of stimulation of the cord.
The ganglia at the base of the brain are affected by opium, causing contraction of the pupil, and disturbance of accom-modation.
The grey matter of the spinal cord is at first, and briefly, stimulated by a moderate dose of opium, reflex excitability being increased, as shown by restlessness in man and convulsions in animals. At the stage of cerebral depression, languor and muscular weakness, of spinal origin, set in, and the subject lies down; but there is not even then complete loss of muscular power and irritability; and even in dangerous poisoning by opium the patient can be marched about, if supported on either side.
Following close upon the convolutions and cord, the great vital centres in the medulla are markedly affected by opium. Vomiting is not uncommon as one of the first effects. The respiratory centre, at first unaffected, is then depressed, the respiratory movements becoming quiet, superficial, and irregular; and death by opium poisoning is due to paralysis of the respiratory centre and arrest of breathing, that is, to asphyxia. The cardiac centre is more resistant to morphia, and is first excited so as to increase inhibition (after an evanescent acceleration); but it is soon depressed, and the pulse rises in frequency. The vascular centre is depressed by opium, but never to a dangerous extent; and even in complete narcosis, when respiration is failing, the blood pressure (pulse) responds to afferent stimuli. The full action of opium on the respiration, heart, and vessels will be immediately described.
We shall presently find that the therapeutical value of the action of opium on the central nervous system lies in the fact that it depresses the perceptive and sensory centres so much earlier and more profoundly than the vital centres in the medulla. Its effect on the pupil, heart, vessels, respiration, and cord are either of little positive value in treatment, or are altogether unfortunate.
The functions of the sensory nerve terminations are lowered or arrested by opium, common sensibility being especially reduced, so that pain cannot be originated; but this peripheral anaesthetic or anodyne effect of morphia given by the mouth is decidedly secondary, both in time and in degree, to its allied action on the sentient centres, and to its local effect when administered by hypodermic or interstitial injection, already described.
The sensory nerve-trunks are diminished in conductivity by local injection of morphia, as well as by its internal administration, thus offering a second interruption to the flow of painful impressions inwards.
The motor nerves are first briefly excited, and then paralysed from the centre outwards. Muscular irritability is never completely lost.
The action of opium upon the centres of several of the viscera has been partly described under the previous heads. In addition to this, it depresses the afferent (including the sensory) nerves of all organs, and acts upon many of the viscera directly.
The heart is temporarily accelerated by opium, in part through the cardiac centre, in part through its intrinsic ganglia. Thereafter, or with fuller doses, it is slowed by stimulation of the vagus in the medulla and heart. Finally, the cardiac vagus is depressed or paralysed; but by this time the intrinsic ganglia are so depressed that acceleration is impossible, and the action remains infrequent, whilst very feeble. Very rarely death occurs by sudden cardiac failure.
The vessels, dilated through the centre, as described, are not directly influenced by opium, either in their muscular coats, or in their peripheral nerves.