In the action of the uterus, the repeats take place more slowly than in either of the in stances just named, although between these two, also, there is a proportionate difference, nearly, or quite as great. The heart's rhythm being quickest, the respiratory rythm slower, and that of the uterus slowest of all.
But the rythm of the uterus does not ob serve a constant or uniform rate. At the commencement of labour, the order of se quence of the rythmic motion remains for a time tolerably constant ; but as the process advances the rythm becomes modified, so that, like the example of the heart under violent emotion, the interval shortens, while the force and vigour of the contractions increase.
It is a matter of great interest to discover, if possible, the determining cause of this rythm • that which constitutes the regulating as well as the disturbing force. The latter should be rather termed the accelerating force, for it is beyond question a healthy- ne .cessity which, for the purpose of advancing the process, demands this graduated change of the uterine ry thm throughout labour. Rythm plainly does not, like peristaltic action, depend upon the structure of the organ which displays it, for the three examples here taken, viz., respiratory muscles, heart, and uterus, differ from each other materially in composi tion. The first consists of striated voluntary fibre ; the second of striated involuntary fibre ; the third of unstriated involuntary fibre. It may therefore be concluded, that something else than structure determines rythm. This appears to depend rather upon the manner in which the contractions are evoked, and hence upon the mode of innervation, which is dif ferent for each organ. The heart and respi ratory muscles each admit of more easy ob servation than the uterus, and referring to them for aid in the elucidation of this ques tion, we find that each of these organs, or sets of organs, is provided with a nervous rythmic centre, upon which its rythm depends, and upon the injury or destruction of which the rythm immediately ceases, — the rythmic centre of respiration being in the medulla ob longata, and that of the heart in its own pro per ganglia. Which of these divisions of the
nervous system furnishes the rythmic centres of the uterus has not been determined, but from the analogies just quoted, we may select by preference the heart, because its actions most nearly resemble those of the uterus, in being purely involuntary, while the case of the respiratory muscles constitutes an ex ample of mixed rnovements wherein volitional can be superadded to unconscious rythmic motion.
If therefore the rythmic action of the uterus is regulated in like manner with that of the heart, we must, upon the strength of this analogy, look for its rythrnic centres among the sympathetic ganglia which lie nearest to the organ.
And this view does not necessarily exclude a certain influence of the spinal nerves over the rythmic action of the uterus. For just as under emotion or bodily excitement both the cardiac and respiratory rythms are accele rated, so, as labour advances, and more parts become irritated, the uterus appears to receive an addition of nerve force which may be pos sibly acquired from other and more distant centres than its own proper ganglia.
The heart's rythmic centres have been re garded by some physiologists as so many " magazines" of nerve-force, whence at regu lated intervals this force is discharged, causing the muscular structure to contract in accord ance with the rate of supply of the stimulus. The influence of these nerve-centres is best shown by placing a ligature upon them, or by cutting them away. When hindered in their operation by tying, the rythm ceases, though the motor power is not lost. When they are cut away, together with certain portions of the heart, the other portions cease to have rythmic motion, though they may still be artificially excited to repeated single ac tions.* But an inconstant stimulus thus furnished to the muscular structure being powerless to produce a permanent or tonic contraction, the effect after a short time passes away to be reproduced upon a fresh application of the excitement. In this way rythm, so far as it is dependent upon nervous supply, is ap parently determined.