ical processes take place with the intervention of colloids, they are very sensitive to changes in hydrogen-ion concentration. This is particularly the case with enzymes. The blood contains sodium bicarbonate, which in itself has an alkaline reaction, owing to hydro- lytic dissociation, together with carbon dioxide which forms an acid when dissolved in water. The reaction or hydrogen-ion concen- tration of the blood is thus controlled by the relative proportion of these two constituents, and is normally at a very slight degree of alkalinity. If acid is formed anywhere and passes into the blood, it combines with part of the bicarbonate, driving off carbon dioxide and thus raising the acidity. But the respiratory centre is enor- mously sensitive to such a rise and removes carbon dioxide by increased ventilation of the lungs until it is reduced to a level cor- responding to the reduction of the bicarbonate. It has been held by some that the proteins assist in the process, but it is very doubt- ful whether this effect is more than very trifling, if present at all.
Excretion of Waste Products. The removal of one of the chief of these, carbon dioxide, has been discussed. The most important of the non-volatile products is urea, most of which is derived from the ammonia of the amino-acids absorbed in excess of the amount required for repair. There are a number of others in small quantity, some nitrogenous, others not, which, although not really of a toxic nature, are of no value. These are all non-colloidal and therefore pass freely through the wall of the blood-vessels. The colloidal constituents of the blood, mainly proteins, and the blood corpuscles do not normally pass through, except in a few places, such as the liver, where the membrane is incomplete. Owing to the pressure produced by the heart, there is a tendency to a continuous filtration through the walls of the blood-vessels of a solution containing all the diffusible constituents of the blood. But the proteins of the blood do not diffuse and, since they are such as to have an osmotic pres- sure of about 30 to 40 mm. of mercury, they may be regarded as attracting water with a force of this magnitude. Unless the blood pressure exceeds this value, therefore, no nitration occurs, and where it is below, reabsorption takes place. The pressure in the arterioles and beginning of the capillaries exceeds the osmotic pressure of the colloids, whereas it is lower than this in the rost of the circulation. In the greater part of the body, the filtered fluid escaping reabsorption is known as lymph. In the kidneys, the glom- eruh are arranged so as to filter a large amount of fluid, which is the first stage of the production of urine. This process as described would suffice to remove all the waste products. But if the whole filtrate were allowed to escape from the kidney, not only would a large quantity of water be lost, but with it such valuable substances as salts and food materials, sugar and amino-acids. What we actually find is that the filtrate is caused to pass along a system of tubules, in the course of which a large part of the water, together with useful solutes, is reabsorbed, while the useless waste products are left in more concentrated solution. It has been shown by Cushny that if it be supposed that the fluid absorbed has invariably the normal composition and concentration of the blood plasma as regards diffusible substances, but without the waste products, all the phenomena of renal function can readily be explained.
It would appear that the chief function of the plasma proteins is to confer a colloidal osmotic pressure, so that excessive filtration is avoided. In the absence of such an osmotic pressure, not only would a large amount of liquid be exuded into the tissues, but there would be no force available for its reabsorption and a dropsical state would result. In fact, this is what happens when a simple salt solution is introduced into the circulation (see SHOCK).
On the basis of the theory given above, it will be noted that the energy for the actual production of the glomerular filtrate is provided by the blood pressure, that is, by the contraction of the heart. _On the other hand, the cells lining the tubules have to do work against osmotic forces, since they remove a dilute solution from a more con- centrated one. This work increases as the fluid passing along the tubules becomes more concentrated. It must be provided by some cellular mechanism analogous to a pump, requiring the provision of energy to actuate it. The investigations of Tamura indicate that this consumption of energy per unit time is unchanged, whatever may be the amount of urine produced by the kidney. Hence, the more concentrated the glomerular filtrate, the less fluid is reabsorbed from it.
Stimulation and Environment. The capacity of an organism to respond in an appropriate manner to changes in its environment clearly depends on its power of properly appreciating such changes. Hence, the more richly endowed is an organism with means by which it is enabled to be affected by the various forms of energy impinging upon it, the better it is fitted to profit, both materially and intellec- tually, by knowledge of the outer world.
In order to understand the essential character of a receptor or sense-organ, as we call the structures by which such information is obtained, one or two fundamental facts brought out more clearly by investigations in recent years have to be considered.
Receptor organs are connected to the brain each by its own set of nerve fibres. These fibres proceed to special regions in the brain, and it appears that whatever the manner in which impulses are set in motion along the fibres, the process in them is the same. So that the fact that the sensations aroused are in one case light, in another taste, and so on, depends on the terminus in the brain. And from
whatever source this terminal " centre " is aroused to action, the effect in consciousness is the same. If, for example, the trunk of a nerve of taste is stimulated, either electrically, mechanically, or chemically, a sensation of taste is evoked. A further fact that has been made clear by Adrian's experiments is that the nerve-impulse itself cannot be made other than of a definite magnitude by varying the strength of the stimulus. In any particular state of the nerve, if it is excited at all, the maximum response possible is obtained. In the case of the heart muscle, this fact has long been known and was given the name of the " law of all-or-nothing " by its discoverer, Bowditch. This law has now been shown to hold for voluntary muscle and for nerve. It is true that the work of Adrian was done on motor nerves, but no difference between these and sensory fibres in other respects has been shown to exist, and there is some direct evidence that the law holds in the case of the optic nerve. It appears then that the impulses travelling along nerves are the same in all cases and that their various results are due merely to the structures in which the fibres end. The statement applies also to efferent nerves, in that there is no difference in nature between nerves which have excitatory and those which have inhibitory action on the structures in which they end. The difference, as Langley pointed out, is in the manner in which their final connexion is made. The question is indeed an aspect of Miillcr'slaw of specific sense-energies. The name is not very explicit, but the law states that the excitation, however produced, of each nerve of special sense gives rise to its own peculiar sensation.
We see then that what is required in a receptor organ is that some process shall be set going in it on the incidence of a particular form of external energy, and that this process shall be such as to stimulate the nerve fibres arising from the organ. It is clear that receptors of a different kind are necessary in the cases of light, sound, heat, touch and so on. While the nerve fibre itself can be stimulated by pressure or by heat of sufficient intensity, it is insensitive to light or sound waves, and even in the former case its sensibility to direct action is far too small for the appreciation of the fine degrees of touch, tem- perature, etc., which is required. The state of affairs is well shown by the properties of the heat and cold spots on the skin. There are distinct receptors for temperatures above that of the skin and below it. The former give a sensation of heat, the latter of cold, and a temperature that feels warm to a heat spot has no effect on a cold spot and vice versa. Thus each is especially sensitive to its own ap- propriate stimulus. If an electrical stimulus or a temperature high enough to affect the nerve fibres directly is used, the sensation from a heat spot is one of warmth, the opposite from a cold spot. But the intensity of stimulus necessary is much greater than that required in the stimulus for which the particular organ is adjusted. The paradoxical fact that a temperature of 45C. feels hot to a heat spot and cold to a cold spot is readily explained on the basis of stimula- tion of the nervesof the organ and the operation of the law of specific sensation. Although it is not easy to prove the fact directly, there is every reason to believe that, at all events in the higher senses, each separate nerve fibre has its own special connexion in the brain and its own individual sensation.
It was remarked above that, so far as evidence goes, all nerve impulses are alike. There is, however, a possibility, pointed out by Keith Lucas, that these impulses may, within the limits imposed by the refractory period, follow one another at different intervals of time. Then, if a particular nerve fibre is connected with two or more neurones in the centre, and if the properties of transmission or re- fractory periods of these " synapses " differ, it may happen that a rapid series of impulses may be able to pass one and not the others. In this way, a single nerve fibre may serve more than one purpose.
For further details the following books may be consulted: Star- ling, Principles of Human Physiology (3rd ed., London, 1020); Bay- liss, Introduction to General Physiology (London, 1919); Bayliss, Principles of General Physiology (3rd ed., London, 1920). In the latter, references will be found in the " bibliography " to special monographs and original papers. (W. M. B.)
PICKERING, EDWARD CHARLES (1846-1919), American physicist and astronomer (see 21.582), died in Cambridge, Mass., Feb. 3 1919.
PINERO, SIR ARTHUR WING (1855- ), English dramatist (see 21.625). Amongst his later plays are The Mind the Paint Girl (1912); The Big Drum (1915); Mr. Livermorc's Dream (1917); The Freaks (1918); and a wordless play, Monica's Blue Boy with accompanying music by Frederic Cowen.
PIRRIE, WILLIAM JAMES, 1ST VISCOUNT (1847- ), British shipbuilder and engineer, was born at Quebec May 31 1847, and educated at the Belfast Royal Academic Institution. In 1862 he entered the shipbuilding firm of Messrs. Harland & Wolff of Belfast, and by his industry and talent he rose to be its head, becoming partner in 1874 and later chairman. His success was particularly associated with the building by his firm of the White Star Line steamships, the first of the line, the "Oceanic," being launched in 1870. From 1896 to 1897 he was
