This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol1", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
It was long an undecided point, whether opium was to be regarded as stimulant or sedative. The experiments of Dr. Crumpe, published in 1793, decided, what any one might have determined for himself by counting his own pulse under a dose of opium, that, in its first operation, it is stimulant at least to the circulation. Within ten or fifteen minutes after its administration, the pulse is, in general, moderately increased in frequency, fulness, and force, and at the same time the surface of the body becomes warmer, and the face somewhat flushed. When the period of general excitement is past, and that of calmness or drowsiness supervenes, the pulse either resumes its original condition as to frequency, or, under a large dose of the opium, becomes somewhat slower, retaining, however, its fulness and for a time its force. In this condition it coutinues for some time during the period of sleep; but then gradually relaxes, and becomes soft with the relaxing surface, and in the end, participates in the general depression which attends the cessation of the direct influence of the medicine.
With the increased frequency of the pulse, the respiration is also somewhat quickened; and, as the former becomes slower, the latter undergoes a similar change, and generally even in a greater degree. Under the full influence of opium, one of the most striking phenomena is the relative slowness of the breathing, which is sometimes even stertorous, when the sleep is profound.
Corresponding with the condition of the circulatory and respiratory movements is that of the blood itself. Retaining its florid colour for a time, it may give a bright tint to the complexion during the stage of excitement; but, with the diminished influence from the respiratory cen-tres, the change from venous to arterial is less thoroughly effected, and the blood becomes darker-hued. This is not very obvious from ordinary doses of opium; but, when it has been very largely taken, the venous hue upon the surface, and particularly in the face, is often conspicuous.