Peristaltic action, as it occurs in vertebrate animals, is found to depend upon the struc ture of the organ displaying it, rather than Upon the mode of its innervation or excite ment. So that if in a situation where organic fibre is usually found, the intestine, for ex ample, of cyprinus, the part is composed of striated muscle, then no organic or peristaltic action can be produced in it ; but upon excite ment, contractions of the kind usually seen in striated muscular fibre ensue.* In the same way the peristaltic action of the uterus although exhibiting certain differences, according to the manner in which it is evoked, is nevertheless to be referred to the peculiar composition of unstriated fibre, and not to the mode of innervation or excitement of the organ.
For the muscular fibre of the uterus is not bound up in separate sheaths, as voluntary muscles are, nor do the fibres run principally in one direction, nor are they long and con tinuous — conditions all favourable to that quick transmission of nerve influence, and rapid action which occur in voluntary muscle but the fibre cells are for the most part distinct, lying in apposition, or imbedded in a matrix of amorphous tissue (fig. 436.), and forming by their combination intricate laminm.
Through a tissue so composed, the in fluence of a stimulus can only be propagated slowly, and the organ formed of it can only contract after a vermicular or peristaltic man ner. Nevertheless, the power, the endurance, and the orderliness of the action that ensues, will be, to a certain extent, dependent upon the nature and mode of application of the excitant. It cannot be questioned that, under many circumstances, the direct application of a stimulus to the uterine muscular structure excites its contractions in the same manner that the food does those of the cesophagus and intestines, without any intervention what ever of nerve. This happens when the hand is passed into the bare uterine cavity after labour, or when the membranes are separated from the inner surface of the uterus by a cath eter.
To bring such an organ into action, all that appears necessary is, that nerve fibres should enter its tissue at a certain number of distinct points or centres, whence the irritation excited at these spots being propagated fi-om fibre to fibre, may spread through the mass, until the whole is brought into harmonious operation.
And it need not excite surprise if these centres of excitement are few, and the nerves of the gravid uterus consequently not nume rous; for a more abundant supply of nerve force, and more rapidly recurring contrac tions, would be prejudicial in labour, by bring ing the uterine walls more constantly and violently into contact with the fcetus, and by driving out the blood passing through them so rapidly as to cause dangerous regurgita tion, or so frequently as to produce fcetal asphyxia, through too constant interruption of the placental circulation.
It is in favour of the iiews of Wigand, who maintains that uterine action begins at the cervix, and travels upwards, that the cervix receives a larger supply of nerves than the fundus, so that the action may be here first established, and the fundus afterwards ex-s cited. But however this may be, it is known that unless all parts of the organ are eventu-s ally brought into consent, the labour does not proceed regularly, for if one portion is felt to be hard, and another at the same time soft, irregular action and spurious pains ensue. To ensure, therefore, consentaneous action between- the respective points of the uterine fibre at which the nerves enter its tissue, and to establish and regulate the movements, ap-s pear to be the offices of the nerves in relation to the uterine structure.
Of the rythinic action of the uterus, and its cause. —The uterus, like the heart and the respiratory muscles, is time-regulated or rythmic in its action. In this action the usual three rythmic periods are noticeable, viz., a period of contraction, a period of re laxation, and one of repose.
The sensible phenomena which accompany the first period are, a gradually increasing and sustained hardness of the uterus, a gradual approach and continuance of suffering, and, after a time, a certain advance of the pre senting part of the child. These occur rences do not commence coincidently, but each overtakes the other in the order enu merated.
The phenomena of the second period are, the gradual subsidence of the hardness, the gradual passing away of the pain, and the re tiring of the presenting part, and these are more nearly coincident than the former.
The third period is marked by an absence of all sensible signs.
These three periods together constitute the uterine rythm, which observes certain laws, that are in some respects different from those which govern the rythmic action of other parts, as for example, of the circulatory and respiratory organs respectively.