Just as stimulus of the sympathetic branches in the rnammalia is followed by acceleration of the heart's action, while stimulus of the pneumogastric causes it to cease pulsating, so also Valentin concludes, from the above ex periments, that stimulus applied to the spinal cord gives rise to the former result, while from stimulus applied to the medulla oblongata the latter result ensues. In the frog, accord ing to Valentin, the spinal cord has no in fluence over the movements of the heart. He also holds, as already stated, that in this animal, the sympathetic, in like manner, exer cises no influence in this respect.
In a pigeon, he found that when the wires of the magneto-electric apparatus were inserted into the cerebellum, the heart's action became more or less laborious : when applied to the spinal cord, in the retion of the first cervical vertebra, forwards, towards the medulla ob longata, the heart's action was repeatedly brought to a stand.
The cessation in the heart's action by appli cation of the galvanic stimulus to the medulla oblongata most readily ensues, according to Valentin, when the wires are applied to its sides, or to the under surface in the vicinity of the roots of the eighth pair, and in no in stance does it ensue when the wires are ap plied to any part of the central nervous masses after removal of the medulla oblongata.
The influence exercised upon the heart's action by the central nervous masses is also shown by the diminution in the number as well as in the strength of its pulsations, which ensues when these are removed, especially on removal of the medulla oblongata. That the diminution in question does not depend en tirely upon the stoppage of the respiratory process consequent on the destruction of the medulla oblongata, has been shown by Budge. When, in the frog, the anterior portion of the medulla is left, the lungs continue to act ; and yet, according to him, the pulsations of the heart diminish very rapidly both in strength and in frequency. He finds that, although removal of the other parts of the central ner vous masses produces little immediate effect on the heart's action, it seldom continues for any length of time after the removal of the medulla oblongata. The effects which follow disease of these parts in like manner illustrate the in fluence vvhich they exercise over the move ments of the heart. In compression of the brain, as well as from lesion of the upper part of the spinal cord, the pulsations are frequently diminished : the effects of shock in altogether stopping its action also illustrate the sante thing.
From the experiments above mentioned, Valentin and others hold that the nervous centre upon which the heart's action depends is the medulla oblongata. The particular rhythmical order in which its different parts contract is due, according to some, to pecu liarities in the manner in which they are acted upon by the blood, the contact of arterial with the lining membrane of the left cavities of the organ, that of venous blood with the lining membrane of those of' the opposite side, fur nishing the proper stimuli, in obedience to which these parts contract. The successive contraction of auricles and ventricles is in like manner explained by the blood first entering the former, and causing them to contract. By their contraction it is propelled into the ven tricles, and stimulates these to contraction also, while the contraction of the ventricles causes the auricles to become again filled with blood from the veins, and so on indefinitely. This rhythmical order in the movements of the organ has also been attributed to pecu liarities in the mode of arrangement of its muscular fibres. The muscular fibres of which
it is composed, as may be seen on examin ing with the microscope the auricles in the heart of the frog, or other small animal, do not lie parallel to one another, as in the ordinary muscles, but cross one another in different di rections, many of the bundles being at the same time observed to present a more or less branching character. The branches or divi sions of one bundle cross those of neighbour ing bundles. In this manner the fibres form a number of reticulated layers laid over one another, while at the same time bundles pass from one layer into the adjacent layers, so that a more or less complete intermixture of the fibres takes place. The fibres composing the ventricles also present more or less of this re ticulate arrangement. Moreover, many of the fibres of the auricles pass into those of the ventricle, and vice versa. In virtue of such an arrangement of the fibres, stimulus applied to one part of the heart gives rise to a contrac tion in the bundle to which it is applied : since this crosses Jieighbouring bundles its con traction acts as a stinaulus to these, in obe dience to which they also contract. In this manner, the contraction is not limited to the fibre, or bundle of fibres, to which the stimulus is first applied, but extends over the entire rnass. So also the contraction of the fibres, which are described as passing between the auri cles and ventricles, stimulate the fibres of which the latter are composed, giving rise to a ge neral contraction in them also ; and in this way the successive contraction of auricles and ventricles is produced. According to Schiff; as mentioned by Valentin, the movements of the heart may be reduced to the peri staltic or vermicular type.. He holds that in a certain part of the muscular substance are contained the nerves which preside over the niovements of neighbouring bundles. When this contracts, a stimulus is thereby given to the nerves which supply the portion of the muscular substance immediately suc ceeding ; so that in this manner a number of progressive contractions of the successive bundles of fibres are produced. The contrac tion of the auricles or ventricles is thus not a single simultaneous act ; but is made up of a great number of contractions succeeding one another, in the same manner as is seen in the contraction of the intestine. It is the rapidity with which they follow one another tbat gives rise to the appearance of their being simul taneous. These contractions travel from auricle to ventricle, giving rise to the successive contractions of these parts. He finds that Ns hen a ring of the muscular substance at the base of the ventricle in the frog's heart is brought, by local application of the galvanic stimulus, into a state of continued or spas nsodic contraction, the due rhy-thm between the contraction of the auricles and the part of the ventricle below the contracted portion ceases. When a spasmodic contraction is produced in a part of the ventricle by ex ternal stimulus this part may be irritated without giving rise to any general contraction. He also finds that when a portion of the ventricle of a heart which still retains its ir ritability, is stimulated, the contraction is sometitnes seen.to take place in this before it takes place in the other portions ; the stimu lated portion is also the part which first be comes relaxed in the diastole of the organ.