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324 DESIGN IN NATURE<br />
PLATE LXXXIV {continued)<br />
Fl«. 4.—Photograph of a frozen section of the human heart where the individual has died during the systole of the organ, that<br />
is, when the ventricles were strongly contracted and the ventricular cavities obliterated ; the auricles being relaxed and distended<br />
to their utmost, a. Cavity of right auricle ; h, cavity of left auricle ; c, thickness of wall of right auricle when relaxed ; d, e, thickness<br />
of wall of left auricle when relaxed ; /, thickness of wall of left ventricle when the ventricle is fully contracted and the<br />
ventricular cavity obliterated ; g, /i, great vessels cut into.<br />
Shows the comparative thickness of the walls of the ventricles and auricles during systole, and how it is impossible for the<br />
thin auricles to force their contained blood into the ventricles, which, in systole, form solid muscular masses. The obvious inference<br />
is that the ventricles open spontaneously when the auricles close. The auricles do not forcibly dilate the ventricles,<br />
neither do the ventricles forcibly dilate the auricles. The movements of the auricles and ventricles are co-ordinated but they are<br />
independent ; the ventricles opening by centrifugal movements when the auricles close by centripetal movements, and vice versa.<br />
The ventricles are invested with a double power, whereby they can alternately open and close, and the same is true of the auricles.<br />
A similar arrangement obtains in the hollow viscera (stomach, bladder, &c.) with sphincters.<br />
All the muscular fibres of the ventricles and auricles move simultaneously and harmoniously in the opening and closing movements.<br />
The opening centrifugal movements and the closing centripetal movements of the ventricles are shown in Pig. 3 (the<br />
Author, 1872).<br />
Fig. 5.—Diagram showing the centripetal and centrifugal movements of the left ventricle during systole, when the ventricular<br />
cavity is obliterated, and during diastole, when the ventricular cavity is fully opened, as seen in cross section of the ventricle. The<br />
muscular fibres are in a state of activity during both the closing and opening movements. There is, however, a brief interval<br />
between the opening and closing movements when the ventricle is at rest. There is also a period of rest for the sarcous elements,<br />
which greatly extends the period of the inactivity of the ventricle as a whole, a, Zone of rest for the left ventricle ; b, contracted<br />
zone when the left ventrical cavity is obliterated by a strong centripetal movement of the muscular fibres, as indicated by the tour<br />
long arrows ; c, relaxed zone when the left ventricular cavity is fully opened up by a more feeble centrifugal movement of the<br />
fibres, as indicated by the four short arrows. The centripetal and centrifugal movements are spontaneous and independent of each<br />
other. They are also vital in their nature (the Author, 1867).<br />
Fig. 6.— Muscles of the iris of the eye. a, Transverse or circular muscular fibres ; b, longitudinal or radiating muscular fibres.<br />
These are arranged at right angles to each other. They are of the plain, unstriped type, and have for their object the increase and<br />
diminution of the pupil or aperture in the centre of the iris through which the light passes into the interior of the eye. The<br />
transverse or circular fibres have been regarded as forming a sjjhincter, and the longitudinal radiating fibres a dilator, to the iris<br />
these acting at different times and alternately. The circular and longitudinal fibres are said to be opposed to, and pitted against,<br />
each other, after the manner of supposed antagonist muscles. This is an erroneous view. As a matter of fact, the so-called<br />
sphincter and dilator muscular fibres are co-ordinated, and act harmoniously and together ; both taking part in the opening and<br />
closing movements of the iris. The movements of the muscular fibres are centrifugal and centripetal in character, as in small<br />
blood-vessels, the hollow viscera, and the voluntary muscles when these produce the movements of flexion and extension, of<br />
abduction and adduction, of pronation and supination, &c.<br />
Some are of opinion that the longitudinal fibres are sparse or non-existent, and that when present they are striated, as in the<br />
bird and otter. If this view be adopted it follows that the circular fibres must of themselves be equal to increasing and diminishing<br />
the aperture of the pupil, and that they are consequently endowed with a double power, which enables them to increase or<br />
diminish the size of the pupil at pleasure (after Iwanoff).<br />
PLATE LXXXV<br />
Plate Ixxxv. shows how the small arteries are furnished with transverse or circular and longitudinal or straight<br />
unstriped muscular fibres arranged in two layers with nuclei interspersed ; how the unstriped muscular fibres are<br />
how the nuclei reappear in young striped muscle<br />
how in the adult heart the transverse marldngs are pronounced, the involuntary<br />
ribbon-shaped and flat, and display rod-hke nuclei in their interior ;<br />
with faint transverse markings ;<br />
and voluntary muscles having not a little in common, structurally and functionally. In particular the Plate shows<br />
how the ventricles of the mammalian heart open and close, the cavities of the ventricles being quite obhterated<br />
during the systole ; how the muscular fibres of the left ventricle are arranged spirally in two sets, which enter and<br />
leave the apex at opposite points, and so produce a symmetrical result, which, curiously enough, finds its exact<br />
counterpart in certain shells (Plate xiii., Fig. 1, G) and in double spiral nebula3 (Plate viii.). The last is a most<br />
unexpected occurrence, and has much significance, as indicating the prevalence of laws which have symmetry and<br />
spiral movements for their objects.<br />
^'"''." ^'*®-^?' ''.°"^'^*i"g °f transverse and longitudinal unstriped<br />
/M ^"'L^'T-f"'<br />
muscular fibres branching into two smaller ones<br />
(/;), which split up into capillaries (c and d) displaying nuclear elements. Magnified 225 diameters<br />
B. Nuclei (ff, (I, a) of unstriped involuntary muscles, magnified 350 diameters.<br />
^<br />
0. Flat spindle-shaped involuntary muscles, showing rod-like nuclei {a, n, a) the central specimen being treated with acetic<br />
according to some investigators, display faint traces of transverse and longitudinal cleavage. Magnified 350<br />
dkmeterT<br />
D, E. Similar fibres with well-marked nuclei (o, (t).<br />
F Elementary fibres from pectoral muscle of fcetal calf two and a half months old, showing corpuscles (a, a), magnified 300<br />
diameters This preparation affords a Imk between the involuntary and voluntary muscular fibres structurally and unctionally.<br />
MagnVd^^rSet^rMBowl^^ '^"-^^ °' ' '-'''"' '' '" ^" ""^''^ ^'^'^ °^ development, showing central row of corpuLles.<br />
H. Striped mvoluntary muscle of the heart. The cardiac muscle is peculiar in this : it is involuntary and yet its<br />
displays we 1-marked transverse<br />
substance<br />
and longitudinal cleavages. It runs the involuntary and voluntary muscle into each<br />
affords proof that both kmds othei ^nd<br />
'<br />
of muscles belong to a common stock, and functionally resemble each other