RHYTHMIC MUSCLES 269 



their parts, up to a point, and within hmits ; they cannot, however, produce new parts. I say within hmits, for no 

 satisfactory proof has yet been adduced to show that plants and animals can modify themselves indefinitely, or that 

 they can merge into each other by insensible gradations, and be all traced back to a common protoplasm or parent. 

 Plants and animals cannot alter their original constitutions ; they have their boundaries in time and space which they 

 may not outrun and survive. We cannot add a cubit to our stature by willing or wishing. The laws of growth and 

 the amount of growth and modification are regulated and determined in each individual case, a remark which 

 applies even to the amount and length of hair on the head and beard of man himself. 



§ 53. Rhjrthmic Muscles— Rhythms not Confined to Involuntary Muscles. 



The rhythmic muscles, as a rule, have their fibres running in two or more directions. In the simplest arrange- 

 ments they consist of two sets, and run at nearly right angles, as seen in the small and large intestine, in the small 

 arteries, and in the iris of the eye, where they are spoken of as longitudinal and circular. Strictly speaking, the 

 circular fibres consist of two sets of very oblique spiral fibres crossing each other at very acute angles. In the 

 smaller arteries, the circular muscular fibres preponderate. In the respiratory muscles, especially the serrati, the 

 muscular fibres run obhquely and cross each other at obtuse angles. In the oesophagus, stomach, bladder, uterus, 

 and heart, the number of muscular fibres, and the variety of directions pursued by them, are increased. Thus in 

 the CESophagus, five or more sets of muscular fibres having varying directions can readily be made out ; while in 

 the stomach, bladder, and ventricles of the higher vertebrate hearts (bird and mammal), the number is increased 

 to seven. The peculiarity of the rhythmic muscles is, that they do not work singly or in sets but collectively and 

 consentaneously. This follows, because while the muscular fibres pursue different directions, and overlap and cross 

 in layers, they are geared to and connected with each other by muscular slips, blood-vessels, nerves, lymphatics, 

 cellular and other tissues. 



The different sets of muscular fibres, crossing as they do in every direction and forming beautiful networks, 

 secure the greatest amount of strength with the least possible material. The hollow viscera, as far as their muscular 

 fibres are concerned, are constructed on strictly mathematical principles. 



In the case of rhythmic muscles strength and efficiency of movement are the chief desiderata, and these 

 are obtained not by the several sets of muscular fibres acting separately and opposing each other, but by 

 their acting simultaneously and in harmony, and by their performing co-ordinated centripetal and centrifugal 

 movements. 



Rhythmic muscles are, in every instance, involuntary, fundamental structures : that is, they work inde- 

 pendently of the will, and are necessary to the life and well-being of the individual. They are, in some senses, 

 more important than the voluntary muscles. Their function is to take in, retain, discharge, and circulate the air, 

 fluids, semi-sohds, and sohds on which, and by which, animals Hve. Thus the chest muscles, aided by the lungs, 

 take in, retain, and circulate pure and discharge impure air : the oesophagus, stomach, intestine, and rectum take 

 in, retain, and circulate food and drink, and discharge the effete or waste products thereof : the bladder takes in, 

 retains, and discharges urine at stated intervals : the uterus takes in, retains, and discharges, at the full period, the 

 fully matured fcetus ; and the heart takes in, retains, and discharges, with \mfailing regularity and in given quan- 

 tities, the blood which it is especially designed to propel. The peculiarity of the hollow viscera as a class is, that 

 they retain as well as circulate the substances with which they deal, whether these be gases, liquids, semi-sohds, or 

 sohds. This distinction is, in some respects, very important, as it shows that the contained substances cannot 

 possibly act as irritants. 



The rhythmic muscles (as more than once stated) are endowed with spontaneous, centripetal, and centrifugal 

 movements : in other words, the contents of these muscles do not act as irritants or artificial stimuh and produce 

 the movements in question. This seems quite certain, and for the following reason. If the contents caused the 

 centripetal closing or expelhng movements, they could not consistently cause the centrifugal opening movements, 

 which are diametrically opposite movements. Moreover, the structures exhibiting rhythmic movements, as ex- 

 plained, are retaining as well as receiving and discharging structures. The structures in question do not eject their 

 contents the instant they are received. On the contrary, some of them retain their contents for seconds, others for 

 hours or days or months. The ventricles of the adult vertebrate heart, for example, retain their contents only until 

 a full measure of blood is received ; the blood being discharged into the lungs and the system from sixty to eighty 

 times per minute. The stomach retains the food for one, or more hours, until it is converted into chyme : the 

 bladder collects and retains the urine from three to six hours or longer : the lower bowel or rectum collects and 

 retains the fseces for twenty-four hours or thereby ; and the uterus retains its living contents for periods varying from 

 three weeks to twenty months. 



