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RHYTHMIC MUSCLES 269<br />
their parts, up to a point, and within hmits ; they cannot, however, produce new parts. I say within hmits, for no<br />
satisfactory proof has yet been adduced to show that plants and animals can modify themselves indefinitely, or that<br />
they can merge into each other by insensible gradations, and be all traced back to a common protoplasm or parent.<br />
Plants and animals cannot alter their original constitutions ; they have their boundaries in time and space which they<br />
may not outrun and survive. We cannot add a cubit to our stature by willing or wishing. The laws of growth and<br />
the amount of growth and modification are regulated and determined in each individual case, a remark which<br />
applies even to the amount and length of hair on the head and beard of man himself.<br />
§ 53. Rhjrthmic Muscles— Rhythms not Confined to Involuntary Muscles.<br />
The rhythmic muscles, as a rule, have their fibres running in two or more directions. In the simplest arrange-<br />
ments they consist of two sets, and run at nearly right angles, as seen in the small and large intestine, in the small<br />
arteries, and in the iris of the eye, where they are spoken of as longitudinal and circular. Strictly speaking, the<br />
circular fibres consist of two sets of very oblique spiral fibres crossing each other at very acute angles. In the<br />
smaller arteries, the circular muscular fibres preponderate. In the respiratory muscles, especially the serrati, the<br />
muscular fibres run obhquely and cross each other at obtuse angles. In the oesophagus, stomach, bladder, uterus,<br />
and heart, the number of muscular fibres, and the variety of directions pursued by them, are increased. Thus in<br />
the CESophagus, five or more sets of muscular fibres having varying directions can readily be made out ; while in<br />
the stomach, bladder, and ventricles of the higher vertebrate hearts (bird and mammal), the number is increased<br />
to seven. The peculiarity of the rhythmic muscles is, that they do not work singly or in sets but collectively and<br />
consentaneously. This follows, because while the muscular fibres pursue different directions, and overlap and cross<br />
in layers, they are geared to and connected with each other by muscular slips, blood-vessels, nerves, lymphatics,<br />
cellular and other tissues.<br />
The different sets of muscular fibres, crossing as they do in every direction and forming beautiful networks,<br />
secure the greatest amount of strength with the least possible material. The hollow viscera, as far as their muscular<br />
fibres are concerned, are constructed on strictly mathematical principles.<br />
In the case of rhythmic muscles strength and efficiency of movement are the chief desiderata, and these<br />
are obtained not by the several sets of muscular fibres acting separately and opposing each other, but by<br />
their acting simultaneously and in harmony, and by their performing co-ordinated centripetal and centrifugal<br />
movements.<br />
Rhythmic muscles are, in every instance, involuntary, fundamental structures : that is, they work inde-<br />
pendently of the will, and are necessary to the life and well-being of the individual. They are, in some senses,<br />
more important than the voluntary muscles. Their function is to take in, retain, discharge, and circulate the air,<br />
fluids, semi-sohds, and sohds on which, and by which, animals Hve. Thus the chest muscles, aided by the lungs,<br />
take in, retain, and circulate pure and discharge impure air : the oesophagus, stomach, intestine, and rectum take<br />
in, retain, and circulate food and drink, and discharge the effete or waste products thereof : the bladder takes in,<br />
retains, and discharges urine at stated intervals : the uterus takes in, retains, and discharges, at the full period, the<br />
fully matured fcetus ; and the heart takes in, retains, and discharges, with \mfailing regularity and in given quan-<br />
tities, the blood which it is especially designed to propel. The peculiarity of the hollow viscera as a class is, that<br />
they retain as well as circulate the substances with which they deal, whether these be gases, liquids, semi-sohds, or<br />
sohds. This distinction is, in some respects, very important, as it shows that the contained substances cannot<br />
possibly act as irritants.<br />
The rhythmic muscles (as more than once stated) are endowed with spontaneous, centripetal, and centrifugal<br />
movements :<br />
in other words, the contents of these muscles do not act as irritants or artificial stimuh and produce<br />
the movements in question. This seems quite certain, and for the following reason. If the contents caused the<br />
centripetal closing or expelhng movements, they could not consistently cause the centrifugal opening movements,<br />
which are diametrically opposite movements. Moreover, the structures exhibiting rhythmic movements, as ex-<br />
plained, are retaining as well as receiving and discharging structures. The structures in question do not eject their<br />
contents the instant they are received. On the contrary, some of them retain their contents for seconds, others for<br />
hours or days or months. The ventricles of the adult vertebrate heart, for example, retain their contents only until<br />
a full measure of blood is received ; the blood being discharged into the lungs and the system from sixty to eighty<br />
times per minute. The stomach retains the food for one, or more hours, until it is converted into chyme : the<br />
bladder collects and retains the urine from three to six hours or longer : the lower bowel or rectum collects and<br />
retains the fseces for twenty-four hours or thereby ;<br />
three weeks to twenty months.<br />
and the uterus retains its living contents for periods varying from