8 PHYSIOLOGY PART I. GENERAL VIEW. nnHE word " physiology " may be used either in a general J_ or in a more restricted sense. In its more general meaning it was used largely of old, and is still occasion ally used in popular writings, to denote all inquiry into the nature of living beings. A very slight acquaintance, however, with the phenomena of living beings shows that these can be studied from two, apparently very different, points of view. The most obvious and striking character of a living being is that it appears to be an agent, performing ac tions and producing effects on the world outside itself. Accordingly, the first efforts of inquirers were directed towards explaining how these actions are carried on, how the effects of a living being upon its surroundings are brought about. And the dissection or pulling to pieces of the material body of a living being was, under the name of ANATOMY (q.v.), regarded as simply an analysis pre paratory and necessary to the understanding of vital actions. But it soon became obvious that this anatomical analysis gave rise of itself to problems independent of, or having only distant relations to, the problems which Morpho- had to do with the actions of living beings. Hence in lgy- course of time a distinct science has grown up which deals exclusively with the laws regulating the form, ex ternal and internal, of living beings, a science which does not seek to explain the actions of living beings, and takes note of these actions only when they promise to throw light on the occurrence of this or that structural feature. Such a science, which is now known under the name of MORPHOLOGY (q.v.), might be carried on in a world in which all living things had, in the ordinary meaning of the word, become dead. Were the whole world suddenly petrified, or were a spell to come over it like that imagined by Tennyson in his " Day Dream," but more intense, so that not only the gross visible movements but the inner invisible movements which are at the bottom of growth were all stayed, the morphologist would still find ample exercise for his mind in investigating the form and struc ture of the things which had been alive, and which still differed from other things in their outward lineaments and internal build. Physio- In its older sense physiology embraced these morpho- logical problems, and so corresponded to what is now called BIOLOGY (q.v.) ; in its more modern sense physiology leaves these matters on one side and deals only with the actions of living beings on their surroundings (the study of these necessarily involving the correlative study of the effect of the surroundings on the living being), and appeals to matters of form and structure only so far as they throw light on problems of action. Looking forward into the far future, we may perhaps dimly discern the day when morphology and physiology will again join hands, and all the phenomena of living beings, both those which relate to form and those which relate to action, will be seen to be the common outcome of the same molecular processes. But that day is as yet most distant ; and, though occasion ally even now the two sciences cross each other s path, action explaining form and form in turn explaining action, the dominant ideas of the two are so distinct, the one from the other, that each must for a long time yet be developed along its own line. It is proposed to treat in the following pages of physiology in this narrower, more restricted sense. If any one at the present day, making use of the know ledge so far gathered in, were to attempt a rough prelimi- fined. nary analysis of the phenomena of action of a living being, for instance of one of the more complex, so-called higher animals, such as man he might proceed in some such way as the following. One of the first, perhaps the first and most striking fact Move- about man is that he moves : his body moves of itself from niellt s place to place, and one part of the body moves on another. muscle If we examine any one of these movements, such as the bending of the forearm on the arm, we find that it is brought about by certain masses of fiesh, called muscles, which from time to time contract, that is, shorten ; and these muscles are so disposed that, when they shorten, and so bring their ends nearer together, certain bones are pulled upon and the arm is bent. Upon further examina tion it will be found that all the gross movements of the body, both the locomotion of the whole body and the movements of parts upon parts, are carried out by the contraction or shortening of muscles. The muscles, together with bones, tendons, and other structures, are arranged in various mechanical contrivances, many of them singularly complex ; hence the great diversity of movement of which an animal or man is capable ; but in all cases the central fact, that which supplies the motive-power, is the contrac tion of a muscle, a shortening of its constituent fibres whereby its two ends are brought for a while nearer together. When, pushing the analysis farther, we attempt to solve the question, Why do muscles contract *? we find that the muscles of the body are connected with what is called the central nervous system by certain strands of living matter Nervov called nerves ; and we further find that, with some few ex- system ceptions, which need not concern us now, the contractions of muscles are brought about by certain occult invisible changes called nervous impulses which travel along these nerves from the central nervous system to the muscles. Hence, when a nerve is severed, the muscle to which the nerve belonged, thus cut adrift from the central nervous system, no longer stirred by impulses reaching it from thence, ceases to contract, and remains motionless and as it were helpless. Pushing the problem still farther home, and asking how these impulses originate in the central nervous system, we find that this central nervous mass is connected, not only with the muscles by means of nerves which, carry ing impulses outward from itself to the muscles and so serving as instruments of movement, are called motor or efferent nerves, but also with various surfaces and parts of the body by means of other nerves, along which changes or impulses travel inwards to itself in a centripetal fashion. Moreover, the beginnings or peripheral endings of these other nerves appear to be so constituted that various changes in the surroundings of the body, or internal changes in the body itself, give rise to impulses, which, thus originated, travel inwards to the central nervous system ; hence these nerves are spoken of as sensory or afferent. Such sensory impulses reaching the central nervous system may forth with issue as motor impulses leading to movement ; but on many occasions they tarry within the central mass, sweeping backwards and forwards along particular areas of its substance, thus maintaining for a while a state of molecular agitation and leading to movement at some subsequent period only. Moreover, we have reason to think that molecular disturbances may arise within the central nervous system apart from the advent, either past or present, of any impulses along sensory nerves. Lastly,
the presence of these molecular agitations in the central