But in the case of the uterus we observe that the rate of the rythm must be to a cer tain extent lintited by the peculiar nature of the uterine fibre. For this, as already show n, is of a kind which cannot be excited to rapidly repeated action like the heart. In this form of fibre the response to the stimulus is slow, and often does not take place until after the excitant is withdrawn. Hence the meaning of that slow repetition of uterine action wbich is observed in ordinary labour.
When this point is further examined, it will be fuund that, according to the degree or kind of excitement employed, the uterine rythm may- be merely accelerated, br a ry thinic may be converted into a more continuous action. The influence of the passage of the child during labour over successive surfaces in quickening uterine action has been already shown. Another example may be drawn from the effects of ergot. When ergot is given by the stomach some time usually elapses before the ergotine mixes with the blood sufficiently to excite the rythmic centres, but that being done, the action is simply augmented, or else occasionally it becomes so violent that the in tervals are obliterated, and one contraction becomes merged in another, so that an inter mittent is converted into a continuous uterine action.
But that which more certainly demonstrates that the rate of the motions, whether rythmic or constant, is dependent on the kind and ex tent or irritation, is the variation in the results obtained by different modes of inducing pre mature labour. If, according to the method of Kiwisch, water is injected simply against the cervix, after several repetitions, rythniic action is slowly excited. If the cervix is dis tended by the introduction of a sponge tent, rythmic action ensues more quickly and cer tainly. But if the first proceeding is so varied that the water, instead of being merely thrown against the cervix, is introduced between the membranes and the uterine walls for a very short distance, so as gently to effect their sepa ration from the inner surface of the uterus, labour is induced with greater certainty and speed than in any other way ; but should the separation be carried still further, some such tumultuous form of labour results as ergot pro duces when acting in the manner just specified.
The uterus acting continuously and very ener getically rather than intermittingly.
Influence of the dilferent nervous centres upon the uterus in parturition. —In the present un settled state of neural physiology, especially in relation to the powers of the different nerve centres, it is scarcely possible to arrive at any satisfactory conclusion regarding the relative degrees of influence which these may he sup posed to exercise over the movements of the uterus. The marked differences of opinion still existing upon this subject* afford sufficient evidence of the uncertainty of the data upon which definite conclusions can be based. In this uncertainty, however, all points of the nervous system are not equally involved.
The amount of influence of the cerebrum upon the act of parturition can be determined vvith tolerable accuracy. That the uterus is in communication with the brain is proved by the fact that the woman is conscious of the fmtal movements, and that she suffers pain when the uterus contracts. Emotion may excite, and may also for a time delay, uterine action. The will cannot operate directly upon the uterus, either in the way of producing, or of restraining its contractions, but a slight in direct influence may be occasionally perceived, when by voluntary changes of posture, or by the use of those muscles which assist labour, the force of the uterus is slightly increased ; or conversely, when, by carefully restraining all such movements and actions, this result is avoided.
It is further shown by the occurrence of labour in cases of complete paraplegia, and also during states of unconsciousness, induced by anmsthetics, that the uterus contracts while it is withdrawn from all volitional and like cere bral influences.
These several examples serve to show to what extent the contractions of the uterus may be influenced by the cerebrum, and also how that influence may be withdrawn. It is obvious that psychical influences are neither necessary nor accessory to the simple act of labour. They may often be regarded as dis turbing, but not as regulating forces. Hence the dominant power over the contractions of the uterus, which is exercised during labour, must have its seat in some of those nervous cen tres that are placed lower than the cerebrum.