PHYSIOLOGY 19 in the protoplasm of the cell, and that in the act of secre tion this trypsinogen is discharged from the cell in the form of the simpler trypsin. When, however, we come to consider the origin of the trypsinogen we pass to matters of inference and to a certain extent of speculation. Two views seem open to us. On the one hand, we may adopt an old theory, once generally accepted, and suppose that the cell picks out from the lymph which bathes it part icles of trypsinogen, or particles of some substance which is readily transformed into trypsinogen, and deposits them in its substance. This may be called the " selective " theory. On the other hand, we may suppose that the trypsinogen results from the breaking down, from the katabolic or de structive metabolism of the protoplasm, being thus wholly formed in the cell. This may be called the " metabolic " theory. Our present knowledge does not permit us wholly to prove or wholly to disprove either of these theories ; but such evidence as we possess is in favour, and increasingly in favour, of the metabolic theory. All efforts to detect in the blood or in the lymph such substances as trypsinogen, or analogous substances in the case of other glands, have hitherto failed ; and, although such a negative argument has its weakness, still it is of avail as far as it goes. On the other hand, the diminution of the protoplasm in the pancreatic cell, pari passu with the increase of trypsinogen, and its subsequent renewal previous to the formation of new trypsinogen, strongly support the metabolic theory, and a number of other facts drawn from the history of various animal and vegetable cells all tend strongly in the same direction. We have further a certain amount of evidence that trypsinogen arises from an antecedent more complex than itself, as it in turn is more complex than trypsin. So, although clear demonstration is not as yet within our reach, we may with considerable confidence conclude that trypsinogen and other like products of secreting cells arise from a breaking down of the cell- substance, are manufactured by the protoplasm of the cell out of itself. We are thus led to the conception that the specific material of a secretion, such as the trypsin of pancreatic juice, comes from the protoplasm of the cell, through a number of intermediate substances, or mesostates as they are called ; that is to say, the complex protoplasm breaks down into a whole series of substances of decreasing com plexity, the last term of which is the specific substance of the secretion. Now the protoplasm is undoubtedly formed at the expense of the material or pabulum brought to it from the blood through the medium of the lymph ; the pabulum becomes protoplasm. Here also two views are open to us. On the one hand, we may suppose that the crude pabulum is at once by a magic stroke, as it were, built up into the living protoplasm. On the other hand, we may suppose that the pabulum reaches the stage of protoplasm through a series of substances of increasing complexity and instability, the last stage being that which we call protoplasm. And here, too f no absolute decision between the two views is possible, but such evidence as we do possess is in favour, and increasingly in favour, of the latter view. We may therefore with considerable confidence anti cipate the future arrival of evidence which will demonstrate the as yet only probable view that in the secreting cell there are two series of events, two staircases, as it were, of chemical transformation, one an ascending staircase of synthetic, anabolic processes through which the pabulum, consisting of several substances, some of them already com plex and unstable, is built up into the still more complex and still more unstable protoplasm ; the other a descending staircase, consisting of a series of katabolic processes giving rise to substances of decreasing complexity and increasing stability. The substances or mesostates appearing in the former we may speak of as "anastates," those of the latter we may call " katastates." At each step of the former, by which a simpler anastate becomes, or by which simpler anastates become, a more complex one, energy is absorbed ; at each step of the latter energy is set free. And, since in the animal-cell the initial anastates seem always or at least generally more complex than the final katastates, the total life of the animal-cell is virtually a giving forth of energy. So far we have spoken of the secreting cell, but we have Mole- evidence that in the activity of a muscle a similar series cular of events takes place. Reduced to theoretical simplicity, changes the unit a number of which go to form a muscle is a proto- plasmic cell, undergoing, like the secreting cell, a con tinual metabolism, with a change in the results of that metabolism at the moment of functional activity. Put in a bald way, the main difference between a secreting cell and a muscle-cell, or elementary muscle-fibre as it is often called, is that in the former the products of the metabolism constitute the main object of the cell s activity, a change of form being of subordinate importance, whereas in the latter the change of form, an increase of one axis at the expense of another, a shortening with corresponding thick ening, is the important fact, the products of the metabol ism which thus gives rise to the change of form being of secondary value. Now we have evidence, which, as in the case of the secreting cell, though not demonstrative, is weighty and of daily increasing weight, that the change of form, the contraction of a muscle, is due to a sudden metabolism, to an explosive decomposition of what may be called "contractile substance," a substance which appears to be used up in the act of contraction, and the consumption of which leads with other events to the exhaustion of a muscle after prolonged exertion. We know as a matter of fact that when a muscle contracts there is an evolution of a considerable quantity of carbonic acid, and a chemical change of such a kind that the muscle becomes acid. This carbonic acid must have some antecedent, and the acidity must have some cause. It is of course possible that the protoplasm itself explodes, and is the immediate parent of the carbonic acid and the direct source of the energy set free in the contraction ; but evidence analogous to that brought forward in relation to the secreting cell leads to the conclusion that this is not so, but that the explosion takes place in, and that the energy is derived from, a specific contractile substance. And there is further evidence that this hypothetical substance, to which the name of " inogen " has been provisionally given, is, like its analogue in the secreting cell, a katastate. So that the contracting activity of a muscular fibre and the secreting activity of a gland-cell may be compared with each other, in so far as in each case the activity is essentially a decom position or explosion, more or less rapid, of a katastate, the inogen in the one instance, the trypsinogen or some other body in the other instance, with the setting free of energy, which in the case of the secreting cell leaves the sub stance wholly as heat, but in the case of the muscle partly as movement, the activity being followed in each case by the discharge from the fibre or cell of the products, or some of the products, of this decomposition. Further, we may carry on the parallel to nervous sub- In ner- stance. As a nervous impulse travels along a nerve-fibre or vous sub- pursues its intricate course along the fibrillar tracts of a s nerve-cell, the amount of chemical change is too slight to be satisfactorily appreciated by the methods at present under our command. There is certainly no massive ex plosion like that of a muscular fibre, and the most striking
phenomena attending the passage of a nervous impulse are