The next experiments related to the effects upon the heart of certain stimuli applied to the brain or spinal marrow. Spirit of wine, applied to the brain or to the upper portions of the spinal marrow, excited considerable increase of the heart's action, but not when applied to the lumbar portion. A watery solution of opium or infusion of tobacco also, occasioned a slight increase; but this was soon succeeded by more languid action of the heart, which, however, recovered its power as soon as these applications were washed off from the brain or spinal marrow.
When tincture of opium is applied to the hind legs of a frog, the animal is deprived of sensibility in less than a minute; but this effect was found not to arise from the opium, but solely from the spirit of wine in which it is dissolved, which alone has the same effect, while a watery solution of opium has no such power. But though a frog be rendered insensible by application of spirit of wine to its feet, the heart nevertheless continues to act, and its force is even increased by subsequent application of spirit of wine to the brain or spinal marrow.
The effects of opium or tobacco were also found to be the same upon frogs as they had been observed in former trials upon rabbits.
In the course of these experiments it was observed, that considerable pressure either on the brain or spinal marrow, had little or no effect on the motions of the heart; and it was further remarked, that the peristaltic motion of the intestines was not affected thereby, and indeed that it in general obeyed the same laws as those of the heart in regard to being influenced by stimuli applied to the brain or spinal marrow, but at the same time that it is not dependent on those parts for their continuance.
Since the apparent inconsistency between two facts, both well ascertained, evidently arises from some imperfection of our knowledge of the principles by which they are to be explained, the author endeavours to elucidate this subject by further experiments.
By applying strong stimuli, and repeating them, to the spinal marrow of a frog, the muscles were made to contract till their irritability was exhausted. In a second frog the nerves supplying the muscles of one leg were divided, and the irritability of its muscles was exhausted by the application of salt to the muscles themselves; and afterwards that of the other leg was exhausted by the same means, without dividing the nerves. Under these different circumstances the irritability was sooner exhausted in that limb to which the nerves remained entire. It appeared therefore, that the property of the heart, of being excitable independent of nervous influence, is common to it with other muscles; and also its property of being excited through the medium of its nerves, although it possesses the latter in a much less degree, being sparingly supplied with nerves, because its usual stimulus is immediately applied to itself, while that of the voluntary muscles is conveyed to them from the sensorium.
And it further appears to the author, that, in the same manner, the spinal marrow is capable of performing its functions independently
