mense columns of blood to and from the brain, yet is itself scantily supplied with nutrient capillaries. But the arterial currents traversing the dura mater, and those derived from other quarters, before they can be admitted into the soft corpuscular parenchyma of the brain, must be divided and subdivided into slender streams, and the impetus of the heart's action subdued by turns and windings. For these ministerial purposes, a more delicate fibrous texture, the pia mater is interposed, which not only affords space and area for the subdivisions of the blood-current, thereby itself becoming extremely vascular, but, dipping down between all the lobular subdivisions of the brain, termed convolutions, it enters the recesses of the organ, supporting the parenchyma, and conveying side by side. arteries and veins through which the blood current reaches and returns from this great organ of life and being. The great vascularity of the pia mater, therefore, is not subservient to any special vital process of secretion in the texture itself, but simply ministerial to the capillary circulation of the corpuscular parenchyma of the brain. So likewise the parenchyma of the lung affords space and area for an extreme subdivision of the blood-current for the purposes of respiration; and its vascularity is analogous to that of the pia mater—a ministerial and not a nutritive vascularity-subservient, not to any process of secretion, but to respiration. The only outlet and inlet to the lung is by the windpipe, and if the air-spaces, which have

been computed to number 3,488,000,000, or if an expanse of membrane equal in area to 1,500 square feet, were a secreting texture, persons would be always coughing and spitting, which we know, in health, is not the case.

SECTION II.

THE PROCESS OF NUTRITION; OR, THE RECIPROCAL ACTION BETWEEN THE BLOOD AND THE TEXTURES.

The growth and preservation of every part of the human structure, and the renewal of its textures and secretions, depend upon a reciprocal action between the blood and the solid parts termed the Process of Nutrition, in which certain general elements withdrawn from the circulating current become part of the special texture. If the newly-withdrawn matter conform in all respects to the special morphological requirement of the part, the process is said to be normal or healthy; but if it varies from it, so as to change the nature or office of the special elements, or alter the quality of the secretion, it is said to be abnormal or unhealthy. From this general statement, it is evident + Lieberkuhn.

* Keil.

that in the process of nutrition by blood, there are two chief phenomena to be attended to, the one, the abstraction of the general elements from the circulating current; the other, the metamorphosis of these elements whereby they incorporate with the special texture. And upon consideration, it is evident that the elements withdrawn from the blood may undergo the required metamorphosis perfectly or imperfectly, or remain in the state in which they are when first separated from the nutrient current.

If the circulation of the blood be observed with the microscope, in vessels of a transparent texture in the living animal, without any previous rude handling or irritation, the stream is seen rapid and uniform, without any check or perturbation, and it is impossible, from the rapidity of the current, to discriminate its cells or corpuscular elements, except that here and there colourless-cells, by clinging to the sides of the vessels, or slowly gliding along them, become discernible. But, if the part under observation be irritated, the regularity of the stream is disturbed in a very remarkable manner, and, as if in consequence thereof, colourless-cells in great multitudes are seen separating themselves from the current,-becoming fixed to the walls of the vessels. Some time after the appearance of this phenomenon, a slender line of colourless matter is more or less visible between the stream of red blood and the solid texture, in which the stationary cells are embedded; so that the irritant, of

whatsoever nature it be, has produced a separation between the colourless and red elements of the blood, which is seen to take place within the living vessels, the red, flowing onward sometimes with the utmost rapidity, the colourless remaining stationary, and forming a new interior coat or wall to the vessels. The nature or meaning of this phenomenon is interpreted by the result, which clearly proves it to be one of increased nutrition, for the irritated texture in a few hours is much thickened, and its elements altered, multiplied, or increased. And the fact clearly shows how the elements of any special texture may become mingled with, or supplanted by, cell-organisms, furnished from the blood.

We are precluded from making this satisfactory and conclusive observation in the living human body, because there is no accessible part sufficiently thin and transparent for the purpose; but we have ample grounds for admitting that in man, analogous irritants are followed by similar results. For, if blood be drawn from the reddened skin of a blister, scarlet-fever, or erysipelas, where new layers of cuticle are forming, or from the neighbourhood of a part discharging pus, an unusual amount of colourless cells may be oberved in it with the microscope.

That the separation of the colourless cells and protoplasma of the blood from the red current, is an ordinary phenomenon of growth or nutrition, appears to be proved by a careful microscopical examination of

the walls of blood-vessels in embryonic structures. Plate 3, fig. 6, represents the appearance of a bloodvessel, in the transparent membrane of a human embryo, that is to say, in a texture not only growing rapidly, but undergoing a metamorphosis from cellular to fibrous. The dark portion a represents the column of red cells in the centre of the vessel. At b is a transparent layer of colourless matter-cells and protoplasma-intervening between the red blood and the proper coat of the vessel c. At c is the proper coat or wall of the vessel composed of nuclei and cells, embedded in fibrils, which in this example had scarcely more coherence than the fibrils of a mucus. At d, still further from the blood column, is the coherent cellular texture of the membrane itself, and it appears clearly, that during growth the whole thickness of the wall of the vessel experiences an alteration.

The example cited is that of a coherent cellular membrane in progress towards a fibrous morphology. The texture nearest to the protoplasma is becoming fibrous, whilst that further from the blood is yet cellular. In a perfectly formed fibrous texture, the elements of the texture and the coats of the vessels are fibrous. If in this condition, unconformable cells accumulate in the protoplasma space b, the coats of the vessel c soon become corpuscular, and the action or growth continuing in this type, the fibrous texture would at last become again corpuscular, thus reverting to its embryoniform condition.