Distribution of Nerves in Muscles.

39

posed. The veins in muscles have numerous valves. Lymphatics have not been traced to any distance into the substance of muscles.

Nerves are distributed to all muscles, but in a larger proportion to some than to others, to the voluntary than to the involuntary: the observations of MM. Prévost and Dumas made it would seem upon parts of the former class, have elucidated a question, upon which hypotheses very opposite to truth were till lately entertained, and tend to establish the curious fact, that nerves are not continuous with muscular fibre. According to the statements of these physiologists, the course of a nerve and of its first branches through a muscle is in tortuous lines, the direction of which appears indifferent; but the minute filaments, in which each branch ends, are observed invariably to traverse the muscular fibres at a right angle and at short distances from each other, and then either to return to the same nerve, or to join a neighbouring branch: thus a nerve terminates in muscles by innumerable delicate loops, or the nervous filaments distributed transversely through muscular substance, communicate equally at either end with the brain or spinal chord. This disposition of parts is not to be observed without difficulty in the opake flesh of warm-blooded animals, but is readily seen in the thin transparent muscles of frogs. Several partial instances of a like nature have been long known to anatomists. The branches of the portio dura are found to unite by slender twigs with those of the three divisions of the fifth nerve in the flesh of the face; and in the tongue the union is equally evident of twigs of the ninth nerve with twigs of the gustatory. It is remarkable that in these familiar instances

40

Action and Relaxation

the junction takes place of sentient nerves with nerves of motion. MM. Prévost and Dumas have not ascertained whether the same relation is observed in the union of the ultimate nervous filaments in muscular substance ©.

All muscular parts during life or shortly after death, may be observed in one of two conditions, which alternate, in action or in repose, in tension or in relaxation.

A muscle, when relaxed, is soft, yields readily to lateral pressure, is easily extended in the line of its fibre, and if previously shortened, on becoming relaxed has a greater or less tendency to lengthen itself, which may possibly depend upon its elasticity alone.

A muscle, when in action, is hard, rigid, resists extension, and has a forcible tendency to shorten in the length of its fibre. The rigidity has no reference to the degree of shortening produced, but depends entirely upon the force employed. If with the elbow slightly bent, and the fore-arm in extreme pronation, an attempt be made against a superior resistance, to bring the arm into the state of supination, the biceps flexor cubiti without shortening becomes harder and more rigid than in extreme flexion of the elbow and supination of the wrist sustained without much effort. The shortening of muscles is an accidental result of their action.

Action may be produced in all muscles during life or soon after death by various stimuli; by mechanical irritation, as upon the simple contact of a new surface, or upon cutting, tearing, or pinching the exposed fibre, by chemical excitement, as upon the application of diluted

Majendie, Journal de Physiologie, tome iii. p. 303.

unattended with a Change in Volume.

41

acid or alkaline fluids, and of most neutral salts, or by electricity.

A muscle in action, if allowed to become shorter, gains exactly in thickness what it loses in length. The ventricular portion of the heart removed from a large dog immediately after death by hanging, was immersed in warm water contained in a glass vessel, which was closed below with a ground glass stopper, and terminated above-in an open vertical tube one-third of an inch in diameter. The ventricles continued alternately to contract and dilate for a sufficient length of time to satisfy me that the height of the water in the tube was unaffected by the varying condition of the muscular fibre.

The change in form, which muscular fibres assume during their action, was remarked by the accurate and sagacious Hales.

"If," says he, "the skin be removed from the belly of a live frog, and the abdomen opened on each side so as that its straight muscles may by drawing a little on one side have a strong focal light cast on the inside of them; if in this posture these muscles be viewed through a good microscope, the parallel fibres of the muscles are plain to be seen, with the blood running alternately up and down between each fibre in capillary arteries so fine that only a single globule can pass them. If the muscle happens to act while thus viewed, then the scene is instantly changed from parallel fibres to series of rhomboidal pinnulæ, which immediately disappear as soon as the muscle ceases to act. It is not easy to get a sight of this most agreeable scene, because that on the action of

d

Anatomical and Physiological Commentaries, vol. i. p. 12.

'42

Change in the Direction of

the muscle the object is apt to get out of the focus of the microscope; but those who are expert in the use of these glasses may readily move them accordingly. I have found small frogs best for this purpose, namely such as are not above a third or a fourth of their full growth. Stimulating the foot of a frog will sometimes make it contract these muscles. The frog must be fixed in a proper frame. If repeated observations were made on the muscles thus in action, it might perhaps give some farther insight into the nature of muscular motion e."

The recent researches of MM. Prévost and Dumas have supplied what was wanting in the observations of Hales, and explain the change in form of each single fibre, from which the preceding appearance results. The ventral muscle of a frog so placed in a frame that a current of the galvanic fluid might at pleasure be directed through it, was examined in a microscope. When excited to contract, the fibres were seen to become bent at numerous angles into zigzag lines. When the stimulus was discontinued, the part regained its former length, and the fibres their straight direction. The angles were observed to be placed at nearly equal distances, and corresponded exactly with the point of intersection of nervous filaments. The same circumstances are stated to have been made out in the muscles of warm-blooded animals, and no less in the muscles of the trunk and limbs than in those of the hollow viscera f. Dr. Hales expected to find in the circulation of the blood the cause of muscular action.

• Hales's Hæmastatics, p. 59.

The nervous in

Majendic, Journal de Phys. vol. iii. p. 301, et seq.

Muscular Fibres when in Action.

43

fluence is the idol of the present day; and MM. Prévost and Dumas trace the force of irritability to this source. Two currents of galvanic fluid moving in the same direction have a mutual attraction: in like manner, say these ingenious physiologists, two currents of the nervous influence (which they seem to identify in this instance with galvanism) or the nervous filaments, which are its conductors, may attract each other. This fanciful supposition, which removes the force of a muscle from its fibre to its nervous filaments, is curiously consistent with the fact, that the only apparent change in the muscular fibre when acting, is its zigzag flexion at those points where it is intersected by nerves.

When the ovary of the frog is full of spawn, the abdominal muscles are extended considerably beyond their habitual length. Upon being detached from the body, when in this condition, they are found to lose at once a third of their accidental elongation: but during this shortening the fibres preserve their straight direction; and only when subsequently excited by galvanism to further action, exhibit the phenomena above described, or become shorter, and are thrown into zigzag lines. The observation has equal interest, whether the first contraction be referable, as MM. Prévost and Dumas are inclined to suppose, to the elasticity of muscular substance, or require to be otherwise explained.

Many phenomena of the same description have been noticed, which are said to result from the tone of muscular parts. If a muscle in its medium state of extension be exposed immediately after death, as for instance, the pectoral muscle in a dog, and be divided transversely, the separate portions instantly recede to some distance