any vital contractile force aiding in this effect? - or whether the vital properties of the blood itself may assist or modify its passage through the extreme vessels? Under any of these views, it is equally certain that the agency of the heart (to which, in the higher classes of animals, all other motive powers are subordinate) must have the same general ratio to each part of the body, whatever the changes occurring in its own rate or vigour of action.*

As respects, again, the quality of the blood, and any influence the variations of the heart's action may have in altering this, though we cannot affirm it to be wholly unaffected, and might even suppose some changes to be made by the rate of motion, yet have we no certain evidence of such alteration; and the general presumption undoubtedly is that it reaches the extreme arteries little, if at all, changed in any essential character.

Nevertheless, the variations in the action of the heart cannot be wholly limited in their effect to changes in the general rate and diffusion of the blood. A different propulsion from the central organ must somewhat alter the proportion in which it reaches parts more or less remote; and even the same proportion of excess or deficiency will produce different effects according to the texture and function of particular organs; a point not sufficiently adverted to in the consideration of this subject. A given excess, for instance, as applied to the liver or kidneys, may have little influence on the system at large; while the same increased action through the brain, or even through some particular portion of it, may not only affect this organ itself, but also produce disturbances of circulation

* In descending through the scale of organised life, the functions of the capillary vessels in circulation become gradually of greater importance; till, arriving at vegetable life, we find them solely effective in this part of the economy of plants.

throughout other and distant parts of the body. This, however, is a secondary effect, taking place through nervous agency. The action of the heart cannot alone, or in direct way, produce those singular translations of blood, respecting which we inquire; and we must go to other causes, beyond that of varying propulsion, for explanation of such changes taking place in the body.

The second condition stated, viz. the action of the extreme vessels, and their relation respectively to the state of the blood and to the nervous system, is of much greater interest to the question before us. The subtle and delicate textures, glandular or otherwise, in which they terminate, fulfil (according to laws still very imperfectly known) all the more important changes in the animal economy. And to this part of the system (a view for which we are first clearly indebted to John Hunter) the physiologist and pathologist must equally and especially look for furtherance of knowledge in their several inquiries. After all the more exact researches of late years, we do but partially comprehend those minute and complex mechanisms by which, in spaces barely accessible to the nicest instruments, the various functions of absorption, secretion, and exudation, as well as the translation of blood from arteries to veins, are all simultaneously going on. What the immensity of creation is to the astronomer or geologist, such are these infinitely small dimensions of matter in space to the physiologist. Presuming, or knowing, that all organization, however minute, such as the many thousand lenses which compose an insect's eye, must be due to the action of distinct vessels, circulating, secreting, and absorbing, we have some vague measure of that exquisite minuteness of fabric and formative action, upon which life in its several parts essentially depends.

The discoveries of Dutrochet and others prove indeed that many of these changes may and do proceed, without the intervention of any continuous vascular structure, and that we need no longer look for the open mouths of vessels as essential to the functions by which one fluid is separated from another. It is still matter of dispute among physiologists whether the capillaries are really membranous tubes, or merely interspaces of tissue, through which the blood finds its passage. But though certain of these minute actions may be submitted to a common physical principle, it is obvious that we must admit essential differences of structure or nervous power, or both, to explain their actual diversities, and the relation of one common fluid to these separate functions. This is obviously true as regards the glandular texture; and even in the instance of the serous membranes, where the tissue appears so much more uniform and simple, the secretions from different membranes of this class vary in the proportion of water, animal and saline matters, they contain; and this more remarkably under the several conditions of health and disease.*

It is further manifest, and now generally recognised, that changes take place in these extreme vessels, or in the tissues of which they form the largest part, independent altogether

*Müller's admirable treatise on the Structure of Glands, as well as the later researches in his great work on Physiology, may be considered to have settled several principles in the general doctrine of secretion, particularly those which regard the relation of vascular structures to the effects they respectively produce.

It would be impossible to refer to all the questions that exist respecting the capillary system, and to the researches (through the microscope, injection, and other means) which have been directed to their solution by the most eminent physiologists of the day. They are indeed among the most important in physiology, as approaching nearest to those ultimate actions on which depend all the functions of life.

of any previous alteration in the heart's action, and often connected with the latter only as an occasional cause of disturbance to it.* This is a point of great importance to the particular subject before us, and in truth to every part of medical science. The capillary vessels, as they are the seat and source of many of the great actions of life in healthy state, so have they like relation to disease in all its obvious forms. Here occur, as far as we can observe, the first morbid changes in fever and inflammation; and there is scarcely a symptom in the progress of such disorders, in which their functions, in one part or other, are not involved. The changes they undergo in all chronic diseases, either are originally, or become, an essential part of the malady. Vitiated states of the blood are testified in similar way; and through this part of the system morbid deposits and absorptions take place, and all the diversities of organic disease are more immediately produced.

The direct relation of the capillaries to the sensorium and nervous system is shown in various ways, which will afterwards be mentioned. At present we have only to notice this part of the circulation in its connexion with the sentient extremities of the nerves; the sensations derived from which are variously altered by the quantity and manner of transmission of blood through the extreme vessels. But beyond this, we must probably look to the capillaries as the first seat of those vital actions which are occasional to the system; as the enlargement of the uterus and its appendages during pregnancy; the swelling of the breasts and secretion of milk afterwards; and others which are too familiar to need recital.

* The experiments of Dr. Philip, Le Gallois, and Serres, have shown that particular injuries of the brain and spinal marrow are capable of altering, or in some cases wholly arresting, the circulation through the capillary arteries.

Any changes in the centre, or larger branches of the vascular system, are undoubtedly subordinate to those which have their locality in the very organs thus determined to particular actions of life.*

Another important relation of the capillaries, requiring notice, is that to the external atmosphere, particularly to the fluctuations of heat and cold. I have spoken on this subject when treating of the influence of weather on the conditions of disease. It is manifestly through the capillaries of the surface that these external causes chiefly act on the body. Their actual effect in disturbing the circulation, and altering the balance between outward and inner parts, is shown in numerous cases, both in health and disease; to which, from their familiarity, it is merely necessary to allude as coming under this part of the subject.

Seeing, then, the numerous and remarkable functions of the capillaries throughout the body, it is certain that we must seek here the source, as well as evidence, of many of those disturbances in the balance of circulation which form the subject of this chapter. Altered or disordered action may be induced in the extreme vessels of any part by causes affecting the whole vascular system; but it often originates in them locally, and becomes thence a source of disturbance to the central organs. And this not merely where a particular function is to be performed, as in the cases just quoted, or in the more frequent and familiar cases where local irritation is applied; but also from other conditions less cognisable within the part itself, altering the quantity and

*The singular case recorded by Dr. Houston, of an acardiac twin fœtus, shows that the capillaries may maintain in some degree, through their proper actions, the general movement of the circulation; one question, amongst others, which has created controversy on the subject.