distinctly acid. A considerable quantity of heat is developed during the progress of the rigor. The electric currents alter in direction and finally disappear.

The period at which rigor comes on and its duration depend on (a) the state of the muscles, and (b) the circumstances under which they are placed at the time of death. All influences which tend to cause death of the tissue induce early rigor of short duration, viz., (1) Prolonged activity-as may be shown in a muscle artificially tetanized, or seen in an animal whose death was preceded by intense muscular exertion-causes rigor to appear almost immediately, and to terminate rapidly. (2) High temperature facilitates the production of rigor in dying muscles; indeed, a temperature not much exceeding that normal to the tissue induces rigor. This form of contraction, which is called heat rigor, is brought about in mammalian muscles by a temperature of about 50° C., and in frog's muscles below 40° C. If, however, the temperature of a muscle be suddenly raised to the boiling point, it is killed, and the chief phenomena of rigor are prevented from occurring. (3) Freezing postpones the changes in the muscles upon which rigor depends. (4) Stretching, or any mechanical excitation which tends to injure the tissue, causes it to pass more rapidly into rigor. (5) The application of water and of a number of chemical substances cause muscles quickly to pass into a state of rigor similar to that which ordinarily follows the death of the tissue. (6) Any stoppage in the blood current normally flowing through a muscle, after some time makes it pass into a state of rigidity like rigor mortis, but this may be removed by allowing the blood to flow freely again through the muscle.

It is generally admitted that rigor mortis depends on the tendency of the muscle plasma to coagulate and give rise to myosin and muscle serum. This is, in most respects, comparable with the coagulation of the blood, and may also depend upon the action of some ferment, of which there is no lack in dead muscle tissue. Compare the paragraph on chemistry, pp. 445, 446.

Most of the phenomena of the process of muscle rigor remind us of the changes already described as occurring in muscle, when

it passes from the passive to the active state.

Thus, the shorten

ing of the fibres, the evolution of heat, and the chemical changes may be said to be identical in contraction and rigor mortis. The electrical changes are, however, very transitory, and the rigor is accompanied by loss of elasticity and irritability. Opacity of the tissue marks its later stages.

Thus, while dying, the muscle tissue may be said to go through a series of events analogous to those which would occur in a prolonged contraction without any period of recuperation. The idea has naturally suggested itself to the minds of physiologists, that the active state of muscle depends upon chemical changes which are the initial steps in the coagulation of the contractile substance, when the muscle is dying. The muscle tissue is supposed to contain a special proteid of extremely intricate and unstable chemical constitution, which is constantly undergoing slow molecular change, and which, if not reintegrated by constant assimilation, would pass into coagulation. Under the influence of stimuli a comparatively sudden and intense molecular disturbance is brought about, which produces shortening of the fibres, and the same chemical changes as precede the coagulation. Before the stage of coagulation appears a chemical rearrangement takes place, the result of which is the reconstruction of the unstable complex proteid. If nutriment be withheld, or if the stimulation be too powerful, the recovery cannot take place, and we find the muscle passing from a state of physiological contraction to one of intense exhaustion, and then to coagulation and death.

UNSTRIATED MUSCLE.

So far reference has only been made to the skeletal muscles, the fibres of which are marked by transverse striations, and whose single contraction is extremely rapid and short. The contractile tissues which carry on the movements in the various organs of the body are not striated fibres, but, as has been already stated, consist of elongated flattened cells with rodshaped nuclei. They occur generally in the form of sheets or layers, forming coats for the organs in which they lie. Their single contraction is slow and prolonged, and is generally trans

mitted from one muscle cell to another as a kind of sluggish wave. They are not capable of passing into a tetanic state of contraction, like striated muscles.

The slowest contraction seems to be that of the muscle cells in the walls of the blood vessels. These remain in a state of partial contraction, which undergoes a brief and temporary rhythmical relaxation. The most forcible aggregate of unstriated muscle elements is met with in the uterus. This organ, which has very exceptional motor powers to perform, contracts in somewhat the same way as the muscles of the blood vessels, but more quickly, and with longer rhythmical intervals of partial relaxation. The muscular wall of the intestine, and the iris, are among the most rapidly contracting smooth muscles.

The chemical properties of the smooth muscle are somewhat similar to those of striated skeletal muscles, and they pass into a state of rigor, while dying, which seems to depend on the same causes as the rigor mortis already described.

CHAPTER XXVI.

THE APPLICATION OF SKELETAL MUSCLES.

The consideration of the many varieties of muscles, and the various modes in which they are attached to the bones that they are destined to move, belongs to the department of practical anatomy, and needs no mention here. As a general but by no means universal rule, a muscle has one attachment which is fixed, commonly spoken of as its origin, and a second, called its insertion, upon which it acts by approximating it to the origin. Muscles usually pass in a straight line between their two attachments, but sometimes they act round an angle by sliding over a pulley, or by means of a small bone in the tendon, like the patella.

The muscles are so attached that they are always slightly on the stretch, and thus, at the moment they begin to contract, they are in an advantageous position to bring their action to bear on the bones which they move. When the contraction ceases, the bones are drawn back to their former position without any sudden jerk or jar.

The muscles act upon the bones as levers, by working upon the short arm of the lever, so that more direct force is required on the part of a muscle than the weight of the body moved; but from this arrangement considerable advantages are gained, viz., that a small contraction of the muscle causes an extensive excursion of the part moved, and much greater rapidity of motion is attained.

All the three orders of levers are met with in the movements of the different bones of the skeleton; often, indeed, all three varieties are found in the same joint, as the elbow, where the simple extension and flexion motions of the biceps and triceps muscles give us good examples (Fig. 194).

The first order of lever is used when the triceps is the power and draws upon the olecranon, thus moving the hand and forearm around the trochlea, which acts as the fulcrum. This is

shown in the upper diagram, in which the hand is striking a blow with a dagger.

The second order comes into play when the hand, resting on a point of support, acts as the fulcrum, and the triceps pulling on the olecranon is the power which raises the humerus, upon which is fixed the body or weight (middle diagram).

The third order may be exemplified by the action of the biceps in ordinary flexion of the elbow. Here the muscle, which is the

Diagrams showing the mode of action of the three orders of levers (numbered from above downward) illustrated by the action of the elbow joint.

power, is placed between the fulcrum— represented by the lower end of the humerus and the weight which is carried by the hand (lower diagram).

The various groups of muscles which are so arranged as to assist each other when acting together, are called synergetic, and those which, when contracting at the same time, oppose each other, are called antagonistic. The same muscles may, in different positions of a joint or in combination with other muscles, have totally different actions, at one time being synergetic and at another antagonistic. Thus, the sterno-mastoid muscle may, in different positions of the head, either bend the cranium backward or forward, and so coöperate with two sets of muscles which are definitely antagonistic to one another.

JOINTS.

The unions between the bones of the skeleton are very varied in function and character. They may be classed as :-

1. SUTURES, in which the bones are firmly united by rugged surfaces without the interposition of any cartilage. They are practically only the lines of union of different bones, which grow together to form a single bone.