Jan. 5, 1883.] 



KNOWLEDGE 



by its movement to mix the food with the digestive fluids. 

 1 need hardly add that muscles give to the varied regions 

 of the body their contour and outline. It is the business 

 of tlie sculptor to delineate with precision the form which 

 the various muscles give to limbs and body under the 

 varying attitudes of action and repose. 



There are two distinct classes or kinds of muscle in our 

 bodies. The fii-st of these is the group of voluntary muscles, 

 whilst the second group is that of involuntary muscles. 

 The former derive their name from the fact that we can 

 move them when we like, and the muscles of face, head 

 and neck, arms, trunk, and legs — that is, the muscles of the 

 body generally — exemplify this tirst set. The "involuntary" 

 muscles are those which are outside the command of the 

 will, and which discharge various important duties, re- 

 i|uiring, so to speak, automatic and regular performance. 

 The muscles of the stomach, which, by their action, mix 

 the food with the gastric juice : the muscles which form 

 part of the walls of blood\essels, and which by their con- 

 traction or expansion produce pallor or blushing of the 

 skin for example ; and the muscles of the bronchial tubes of 

 the lungs, and of the pupil of the eye, all illustrate the 

 " involuntary " class. They act only when stimulated by 

 some special feature of life. Light will produce contraction 

 of the special eye muscles whose function it is to close the 

 pupil ; and food entering the stomach sets its muscles 

 agoing. The heart is likewise an involuntary muscle, dis- 

 charging, when left to itself and not interfered with by 

 the brain, its important duty with regularity and precision. 



There is a wide difference in structure between the 

 " voluntary " and " involuntary " muscles. The former 

 are often called striped muscles, and the latter unslriped; 

 but it must be borne in mind that the lieart is an exception 

 to this classLtication. The fibres of the heart are striped 

 like those of the " voluntary " muscles, whilst, of course, 

 the heart is entirely in\oluntary in its action. In some 

 respects the heart-tibres, however, are different from those 

 of the striped muscles at large. 



If we take a " voluntary " muscle, separate it from its 

 surrounding neighbours, and examine it carefully, we find 

 it to be composed of Inindlis of fibres or fasciculi as they 

 are technically named. Each bundle consists of a variable 

 number of fibres (Fig. 1. a.) which at their longest do not 

 exceed one and a half inches in length. The average 

 breadth of a fibre varies from the four-hundredth of an 

 inch to the twenty-four-hundredth. In cold-blooded 

 animals, the fibres are larger than in warm - blooded 

 forms. When we put a fibre of voluntary muscle 

 under the microscope, we observe that, as the light 

 shines through it, it presents a striped appearance 

 (Fig. 1. c), the stripes running across the fibre. This 

 " striped '' appearance, from which the voluntary muscles 

 derive their name, is due to the fact that a fibre is 

 composed of two kinds of elements, thick and thin. 



These alternate with each other, and the thick parts, of 

 course, under transmitted light, appear as the dark stripes, 

 whilst the thinner parts form the intervening light bands 

 of the fibre. The light bands refract the light singly, whereas 

 the dark discs or parts refract the light in a double fashion. 

 More recently, another stripe has been described cutting 

 as it were, each dark stripe into two halves. It is evident, 

 however, that wlien the ultimate structure of a fibre of 

 muscle is considered, we might readily enough conclude 

 that the fibre was really built up of cross-pieces or discs, 

 placed end to end, like a rnulcau of shillings. And, as a 

 matter of fact, we can see this structure in a muscular 

 fibre which has been allowed to soak for some time in a 

 weak acid. As shown in Fig. 2, the filire then breaks into 

 cross-pieces, named its discs (a, h). Again, on the surface of 

 the fibre we can detect markings or stripes running the long 

 way of the structure. This shows us that in another sense 

 the fibre can be divided into smaller fibres or fibrils ; and 

 when the extremity of a fibre is microscopically examined, 

 we may see the ends of these fibres (Fig. 2, c), reminding 

 us of a section of the separate wires which compose a thick 

 wire rope. Last of all, we must note that each fibre is. 

 enclosed in a delicate sheath, called its sarcolemma. 





The unstriped or involuntary muscles possess, as their 

 name implies, a different structure from their striped neigh- 

 bours. They are also composed of fibres, but each fibre is 

 made up of long spindle-shaped cells, varying in length 

 from the three-hundredth to the six-hundredth of an inch. 



The question, finally, " how does muscle act 1 " faces us 

 at the close of our brief studies. If we take the well known 

 biceps muscle, which forms the great fleshy mass of the 

 upper arm, as a type of muscles at large, we may discover a 

 ready reply to this (juestion. The biceps springs from the 

 shoulder by its two tendons (or sinews) and passes, to be 

 attached or " inserted " also by a tendon, into the radius 

 (one of the fore-arm bones) below. Now, when this muscle 

 acts, we see that it pulls up the fore-arm and flexes or 

 bends it on the upper arm, as in the act of bringing food to 

 the mouth. If we place our hand over the biceps (at the 

 middle of the upper arm) as we raise our fore-arm, we shall 

 feel the muscle to grow thicker as the fore-arm approaches the 

 upper arm. Again, we observe that only one end of the 

 muscle moves (the lower end, or insertion), whilst its upper 

 end (or oriyin) at the shoulder, remains fixed. From these 

 facts, then, we learn, firstly, that muscles act by pvdling 

 together the parts between which they are attached ; 

 secondly, that the muscle grows thicker when it acts ; and 

 thirdly, that muscles perform their functions of moving 

 the bones and body, because they possess an inherent 

 property called contractility — that is, the poin-r of shorteniny 

 themselres. In this latter phrase is found the whole ex- 

 planation of muscular action. And when we add that 

 groups of muscles (such as those which bend the fingers), 

 antagonise or oppose others (such as those which open the 

 fingers) ; and that the muscles are ordered and governed by 

 the nervous system, we shall have fairly started our readers 

 on the way of becoming better acquainted from the pages of 

 any physiology text-book with the interesting problem con- 

 cerning our movements and the power of doing as we w^iU. 



