CHEMICAL, PHYSICAL AND VITAL PROPERTIES.

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especially to the striped variety, but, so far as is known, it is essentially the same in the non-striated tissue.

The juice expressed from a muscle after death, and especially after rigidity has set in, is acid, from the presence of lactic acid; so that the cut surface of a dead muscle reddens litmus-paper. On the other hand, a perfectly fresh section of muscle in the living body, or while it retains its irritability, is alkaline or neutral. But while this is true of a living muscle in its usual state, it gives a decided acid reaction after it has been strongly exerted, as, for instance, after tetanic spasm excited by electricity or by strychnia poisoning. The acid is probably generated by a change in the saccharine matter of the muscle. Ultimately the tissue in all cases becomes alkaline from putrefaction and the evolution of ammonia.

Physical properties of muscle.-A dead muscle has little strength, and may be torn asunder by a force of no great amount. A living muscle readily yields to extension, and shrinks exactly to its original length when the extending force ceases. Its elasticity is therefore said to be small in degree, but very perfect or complete in operation. A dead muscle, especially after cadaveric rigidity has come on, resists extension more powerfully, but does not afterwards return to its original length; hence its elasticity is said to be greater than that of the living muscle, but less perfect.

The red colour of muscle is well known, but it differs greatly in degree in different cases. It is usually paler in the involuntary muscles; but here the heart again is a striking exception. In most fish the chief muscles of the body are nearly colourless, and in the breast of wild fowl we see a difference in the depth of colour in different strata of the same muscles. The redness is no doubt partly due to blood contained in the vessels, but not entirely so, for a red colouring matter, apparently of the same nature as that of the blood, is obviously incorporated with the fibres.

Under this head must also be mentioned the manifestation of electricity by a quiescent but living muscle. When a muscle taken from a living or recently killed animal (a frog is commonly used) is brought into connection with the ends of a very delicate galvanometer, so that one extremity of the latter touches the outer surface of the muscle, and the other a cross section made through its fibres, the needle will deviate so as to indicate an electric current passing along the wire from the surface of the muscle to its cross section. If both ends of the galvanometer touch points in the length of the muscle equidistant from its middle, no effect ensues, but if one point of contact be farther than the other from the middle, a current will pass along the wire from the nearer to the more distant point. The same results are obtained with a small shred or fasciculus of the muscle. The phenomenon described is called "the muscular current," and is supposed to indicate a state of electric polarity in the particles of the muscle, probably caused by chemical changes going on in its substance.

Vital properties of muscle.-The muscular tissue possesses a considerable degree of sensibility, but its characteristic vital endowment, as already said, is irritability or contractility, by which it serves as a moving agent in the animal body.

Sensibility. This property is manifested by the pain which is felt when a muscle is cut, lacerated, or otherwise violently injured, or when it is seized with spasm. Here, as in other instances, the sensibility belongs, properly speaking, to the nerves which are distributed through the tissue, and accordingly when the nerves going to a muscle are cut, it forthwith becomes insensible. It is by means of this property, which is sometimes called the "muscular sense," that we become conscious of the existing state of the muscles which are subject to the will, or rather of the position and direction of the limbs and other parts which are moved through means of the voluntary muscles, and we are thereby guided in directing our voluntary movements towards the end in view. Accordingly, when this muscular sense is lost, while the power of motion remains,-a case which, though rare, yet sometimes occurs, -the person cannot direct the movements of the affected limbs without the guidance of the eye.

Irritability or Contractility.—The merit of distinguishing this property of the animal body from sensibility on the one hand, and from mere mechanical phenomena on the other, is due to Dr. Francis Glisson, a celebrated English physician of the seventeenth century; but irritability, according to the view which he took of it, was supposed to give rise to various other phenomena in the animal economy besides the visible contraction of muscle, and his comprehensive acceptation of the term has

been adopted by many succeeding authorities, especially by writers on pathology. Haller, in his use of the term irritability, restricted it to the peculiar property of muscle.

Stimuli. In order to cause contraction, the muscle must be excited by a stimulus. The stimulus may be applied immediately to the muscular tissue, as when the fibres are irritated with a sharp point; or it may be applied to the nerve or nerves which belong to the muscle: in the former case, the stimulus is said to be "immediate," in the latter, "remote." The nerve does not contract, but it has the property, when stimulated, of exciting contractions in the muscular fibres to which it is distributed, and this property, named the "vis nervosa," is distinguished from contractility, which is confined to the muscle. Again, a stimulus may be either directly applied to the nerve of the muscle, as when that nerve is itself mechanically irritated or galvanised; or it may be first made to act on certain other nerves, by which its influence is, so to speak, conducted in the first instance to the brain or spinal cord, and then transferred or reflected to the muscular nerve.

The stimuli to which muscles are obedient are of various kinds; those best ascertained are the following, viz.: 1. Mechanical irritation of almost any sort, under which head is to be included sudden extension of the muscular fibres. 2. Chemical stimuli, as by the application of salt or acrid substances. 3. Electrical; usually by means of a galvanic current made to pass through the muscular fibres or along the nerve. 4. Sudden heat or cold; these four may be classed together as physical stimuli. Next, mental stimuli, viz.: 1. The operation of the will, or volition. 2. Emotions, and some other involuntary states of the mind. Lastly, there still remain exciting causes of muscular motions in the economy, which, although they may probably turn out to be physical, are as yet of doubtful nature, and these until better known may perhaps without impropriety be called organic stimuli; to this head may be also referred, at least provisionally, some of the stimuli which excite convul. sions and other involuntary motions which occur in disease.

Duration of irritability after death.-It is known that if the supply of nutrient material be cut off from a muscle by arresting the flow of blood into it, its contractility will be impaired, and soon extinguished altogether, but will, after a time, be recovered again if the supply of blood be restored. The influence of the blood supplied to muscles in maintaining their contractility has been strikingly shown by Dr. Brown-Séquard, who has succeeded in restoring muscular contractility in the bodies both of man and animals some time after death, and after it had become to all appearance extinct, by injecting into the vessels arterial blood deprived of its fibrin, or defibrinated venous blood previously reddened by exposure to the air. In warmblooded animals in which the nutritive process is more active, and the expenditure of force more rapid, the maintenance of irritability is more closely dependent on the supply of blood and the influence of oxygen, so that it sooner fails after these are cut off. In accordance with this statement it is known that while the muscles of man and quadrupeds cease to be irritable within a few hours after death, and those of birds still sooner, the muscular irritability will remain in many reptiles and fish, even for days after the extinction of sensation and volition, and the final cessation of the respiration and circulation,-that is, after systemic death. A difference of the same kind is observed among warm-blooded animals in different conditions; thus irritability endures longer in new-born animals than in those which have enjoyed respiration for some time and are more dependent on that function; and in like manner, it is very lasting in hybernating animals killed during their winter sleep.

But the duration of this property differs also in different muscles of the same animal. From numerous careful observations Nysten concluded that in the human body its extinction takes place in the following order, viz. 1, the left ventricle of the heart; 2, the intestines and stomach; 3, the urinary bladder; 4, the right ventricle; in these generally within an hour; 5, the gullet; 6, the iris; 7, the voluntary muscles, a, of the trunk, b, of the lower, and c, the upper extremities; 8, the left auricle, and, 9, the right auricle of the heart, which last was on this account styled by Galen the "ultimum moriens." In one case Nysten observed the right auricle to continue irritable for sixteen hours and a half after death. But it has been recently found that a voluntary muscle will give signs of a certain degree of

CADAVERIC RIGIDITY.

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irritability even later than this, if it is struck a smart blow with a blunt edge, such as the back of a knife, across the direction of the fibres. The contraction then produced is quite local, and confined to the part struck. Funke states that he and the brothers Weber obtained this result in the body of a decapitated criminal twenty-four hours after death.

The time of duration is affected by the mode of death. Thus the irritability is said to be almost wholly and immediately extinguished by a fatal stroke of lightning, and to disappear very speedily in the bodies of persons stifled by noxious vapours, such as carbonic acid, and especially sulphuretted hydrogen. In like manner certain causes acting locally on muscles accelerate the extinction of their irritability.

Rigor mortis.-The "cadaveric rigidity," or stiffness of the body, which ensues shortly after death, is a phenomenon depending on the muscles, which become fixed or set in a rigid state, so as to resist flexion of the joints. The rigidity almost invariably begins in the muscles of the lower jaw and neck, then invades those of the trunk, and afterwards those of the limbs,-the arms usually before the legs. After persisting for a time, it goes off in the same order. It usually comes on within a few hours after death, rarely later than seven hours. In some cases it has been observed to begin within ten minutes (Sommer), and in others not till sixteen or eighteen hours; and the later its access, the longer is its endurance. The rigidity comes on latest, attains its greatest intensity, and lasts longest in the bodies of robust persons, cut off by a rapidly fatal disease, or suddenly perishing by a violent death; in such cases it may last six or seven days. On the other hand, it sets in speedily, is comparatively feeble, and soon goes off in cases where the body has been much weakened and emaciated by lingering or exhausting diseases; also in newborn infants, and in the muscles of animals that have been hunted to death. It seems thus to be affected by the previous state of nutrition of the muscles. Destruction of the nervous centres does not prevent the occurrence of rigidity, nor are the muscles of paralysed limbs exempted from it, provided their nutrition has not been too deeply affected. The fibres of stiffened muscles are less translucent than before, but no other change is discovered by the microscope. They no longer show the muscular electric current.

The immediate cause of the muscular rigidity is doubtful: some conceive it to be an effect of vital contraction,-the last effort of life as it were; others, with more probability, ascribe it to a solidification of the tissue caused by chemical changes occurring after death. Kühne adduces various arguments, some of them, it must be admitted, of a cogent character, to show that the stiffening is due to post-mortem coagulation of the myosine. He thinks that the substance of the fibre is liquid during life; but it is difficult to reconcile this notion of actual fluidity of substance with some of the most obvious properties of a living muscle. At the same time, it is conceivable that liquid myosin may be present in the interstices of more consistent elements of the living fibre, and may give rise to rigidity by coagulating after death. Free lactic acid is developed in the substance of rigid muscle, and some regard it as the cause of the coagulation of the myosin, but although an acid condition very generally accompanies rigidity, the concurrence is not invariable or essential. Dr. Brown-Séquard, in opposition to the chemical theory, maintained that he could remove rigidity by injecting blood into the vessels of the muscle; but Kühne holds this to be impossible after rigor has decidedly set in. The general accession of rigidity is an unequivocal sign of death.*

NERVOUS SYSTEM.

Or the functions performed through the agency of the nervous system, some are entirely corporeal, whilst others involve phenomena of a mental or psychical nature. In the latter and higher class of such functions are first to be reckoned those purely intellectual operations, carried on through the instrumentality of the brain, which do not immediately arise from an

The subject of muscular contraction and other questions relating to the functional activity of muscle, treated of in former editions of this work, have outgrown the space that could be allotted to their consideration here, and as, moreover, they properly belong to a treatise on physiology, they have now been omitted.

external stimulus, and do not manifest themselves in outward acts. To this class also belong sensation and volition. In the exercise of sensation the mind becomes conscious, through the medium of the brain, of impressious conducted or propagated to that organ along the nerves from distant parts; and in voluntary motion a stimulus to action arises in the brain, and is carried outwards by the nerves from the central organ to the voluntary muscles. Lastly, emotion, which gives rise to gestures and movements varying with the different mental affections which they express, is an involuntary state of the mind, connected with some part of the brain, and influencing the muscles through the medium of the nerves.

The remaining functions of the nervous system do not imply necessary participation of the mind. In the production of those movements, termed reflex, excited, or excito-motory, a stimulus is carried along afferent nervefibres to the brain or spinal cord, and is then transferred to efferent or motor-nerve-fibres, through which the muscles are excited to action; and this takes place quite independently of the will, and may occur without consciousness. The motions of the heart, and of other internal organs, the contraction of the coats of the blood-vessels, as well as the invisible changes which occur in secretion and nutrition, are in a certain degree subject to the influence of the nervous system, and are undoubtedly capable of being modified through its agency, though, with regard to some of these phenomena, it is doubtful how far the direct intervention of the nervous system is necessary for their production. These actions, which are all strictly involuntary, are, no doubt, readily influenced by mental emotions; but they may also be affected through the nerves in circumstances which entirely preclude the participation of the mind.

The nervous system consists of a central part, or rather a series of connected central organs, named the cerebro-spinal axis, or cerebro-spinal centre ; and of the nerves, which have the form of cords connected by one extremity with the cerebro-spinal centre, and extending from thence through the body to the muscles, sensible parts, and other organs placed under their control. The nerves form the medium of communication between these distant parts and the centre. One class of nervous fibres, termed afferent or centripetal, conduct impressions towards the centre,-another, the efferent or centrifugal, carry motorial stimuli from the centre to the moving organs. The nerves are, therefore, said to be internuncial in their office, whilst the central organ receives the impressions conducted to it by the one class of nerves, and imparts stimuli to the other, -rendering certain of these impressions cognisable to the mind, and combining in due association, and towards a definite end, movements, whether voluntary or involuntary, of different and often of distant parts.

Besides the cerebro-spinal centre and the nervous cords, the nervous system comprehends also certain bodies named ganglia, which are connected with the nerves in various situations. These bodies, though of much smaller size and less complex nature than the brain, agree, nevertheless, with that organ in their elementary structure, and to a certain extent also in their relation to the nervous fibres with which they are connected; and this correspondence becomes even more apparent in the nervous system of the lower members of the animal series. For these reasons, as well as from evidence derived from experiment, but which is of a less cogent character, the ganglia are regarded by many as nervous centres, to which impressions may be referred, and from which motorial stimuli may be reflected or emitted; but of local and limited influence as compared with the cerebro-spinal centre, and

CHEMICAL COMPOSITION.

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operating without our consciousness and without the intervention of the will*

The nerves are divided into the cerebro-spinal, and the sympathetic or ganglionic nerves. The former are distributed principally to the skin, the organs of the senses, and other parts endowed with manifest sensibility, and to muscles placed more or less under the control of the will. They are attached in pairs to the cerebro-spinal axis, and like the parts which they supply are, with few exceptions, remarkably symmetrical on the two sides of the body. The sympathetic or ganglionic nerves, on the other hand, are destined chiefly for the viscera and blood-vessels, of which the motions are involuntary, and the natural sensibility is obtuse. They differ also from the cerebro-spinal nerves in having generally a greyish or reddish colour, in their less symmetrical arrangement, and especially in the circumstance that the ganglia connected with them are much more numerous and more generally distributed. Branches of communication pass from the spinal and several of the cerebral nerves at a short distance from their roots, to join the sympathetic, and in these communications the two systems of nerves mutually give and receive nervous fibres; so that parts supplied by the sympathetic be also in nervous connection with the cerebro-spinal centre.

may

The nervous system is made up of a substance proper and peculiar to it, with inclosing membranes, nutrient blood-vessels, and supporting connective tissue. The nervous substance has been long distinguished into two kinds, obviously differing from each other in colour, and therefore named the white, and the grey or cineritious.

CHEMICAL COMPOSITION.

The information we possess respecting the chemical composition of nervous matter is for the most part founded on analyses of portions of the brain and spinal cord; but the substance contained in the nerves, which is continuous with that of the brain and cord, and similar in physical characters, appears also, as far as it has been examined, to be of the same general chemical constitution. No very careful comparative analysis has yet been made of the grey and white matter, to say nothing of the different structural elements of the nervous substance; and indeed it must be remembered, that, in portions of brain subjected to chemical examination, capillary blood-vessels, connective, and perhaps other accessory tissues, as well as interstitial fluid, are mixed up in greater or less quantity with the true nervous matter, and must so far affect the result.

The nervous matter may be said to consist of an albuminoid body, in part liquid, with fatty principles, extractive matters, salts, and much water. The water, which forms from three-fourths to four-fifths or more of the whole cerebral substance, may be removed by immersion in alcohol and evaporation. When the solid matter which remains after removal of the water is treated with ether and hot alcohol, the fatty compounds are extracted from it by these menstrua, and there remains a mixture of coagulated albuminous matter and salts, with a small remnant due to accessory tissues, chiefly vessels.

The albuminoid constituent is not sufficiently known to be characterised specifically. It no doubt belongs, in some small proportion, to the interstitial fluid. Of that which * From the researches of Dr. Augustus Waller it appears probable that ganglions exert some influence over the nutrition of the nerve-fibres connected with them, and serve to maintain the structural integrity of these fibres; for it has been found that when a ganglionic nerve is cut across in a living animal, the part beyond the section after a time becomes atrophied, while the part connected with the ganglion retains its integrity.