2dly. Mucous membranes line all the outlets of the body,-the airpassages, alimentary canal, urinary and genital organs, &c.

3dly. Fibrous membranes form tendon, aponeurosis, ligament, &c. Compound membranes are formed by the union of the simple, and are divided into fibro-serous, as the pericardium; sero-mucous, as the gall-bladder, at its lower part; and fibro-mucous, as the ureter.

11. Areolar, cellular or laminated tissue-to be described presently -is a sort of spongy or areolar structure, which forms the framework of the solids; fills up the spaces between them, and serves at once as a bond of union and separation.

12. A viscus is the most complex solid of the body; not only as regards intimate organization, but use. This name is given to organs contained in the splanchnic cavities,-brain, thorax, and abdomen,-and hence the viscera are termed cerebral, thoracic, and abdominal.

Every animal solid is either amorphous or fibrous; that is, it is either without apparent arrangement, like jelly; or is disposed in minute threads, called fibres. The disposition of these threads, in different structures, is various. Sometimes, they retain the form of threads; at others, they have that of laminæ, lamellæ, or plates. Accordingly, when we examine any animal solid, where the organization is perceptible, it is found to be either amorphous, or fibrous and laminated.

This circumstance led the ancients to endeavour to discover an elementary fibre or filament, from which the various organs might be formed. Haller' embraced the idea, and endeavoured to unravel every texture to this ultimate element, which, he conceived, is to the physiologist what the line is to the geometer; and, as all figures can be constructed from the line, so every tissue and organ of the body may be built up from the filament. Haller, however, admitted that this elementary fibre is not capable of demonstration, and that it is visible only to the "mind's eye," "invisibilis ea fibra, quam solâ mentis acie adtingimus." It must be regarded, indeed, as a pure abstraction; for, as different animal substances in the mass have different proportions of carbon, hydrogen, oxygen, and nitrogen, it is fair to conclude that the elementary fibre must equally differ in the different substances.

The ancients believed that the first product of the elementary fibre was areolar tissue; and that this tissue forms every organ of the body, the difference in the appearance of the organs arising from the different degrees of condensation of its lamina. Anatomists, however, have been unable to reduce all animal solids to areolar tissue only.

In the upper classes of animals, three primary fibres or tissues or anatomical elements are usually admitted, the areolar, cellular or laminated; the muscular; and the nervous, pulpy or medullary.

1. The areolar, cellular, mucous, filamentous or laminated fibre or tissue is the most simple and abundant of animal solids. It exists in every organized being; and is an element of every solid. In the enamel of the teeth only it has not been detected. It is formed of an assemblage of thin laminæ, of delicate, whitish, extensible filaments,

'Elementa Physiologiæ, vol. i. lib. i. sect. i. p. 7, Lausan., 1757.

interlacing and leaving between each other areolæ or cells. These filaments-although possessed, like every other living tissue, of contractility or the power of feeling an appropriate irritant and of moving responsive to such irritant-do not move perceptibly under the influence of mechanical or chemical stimuli. They are mainly composed of concrete gelatin.-The great bulk of animal solids consists of areolar tissue, arranged as membrane.

2. Muscular fibre or tissue is a substance of peculiar nature; arranged in fibres of extreme delicacy. The fibres are linear, soft, grayish or reddish, and manifestly possessed of contractility or irritability; that is, they move very perceptibly under the influence of mechanical or chemical stimuli. They are composed, essentially, of fibrin. Their histology will be described hereafter.

Muscular fibres, which are arranged in the form of membranous expansions or muscular coats, differ from proper muscles chiefly in the mechanical disposition of the fibres. The physical and chemical characters of both are identical. The fibres, instead of being collected into fasciculi, are in layers, and, instead of being parallel, interlace. This tissue does not exist in the zoophyte.

3. Nervous, pulpy, or medullary fibre or tissue, which will be referred to hereafter, is much less distributed than the preceding. It is of a pulpy consistence; is composed essentially of albumen united to a phosphuretted fatty matter; and is the organ for receiving and transmitting impressions to and from the nervous centres. Of it, brain, cerebellum, medulla spinalis, nerves and their ganglia are composed. Professor Chaussier1 added another primary fibre or tissue, the albugineous. It is white; satiny; resisting; of a gelatinous nature; and constitutes tendons and tendinous structures. Chaussier is, perhaps, the only anatomist that admits this tissue. Others properly regard it as a condensed variety of the areolar.

These various fibres or tissues, by uniting differently, constitute the first order of solids; and these, again, by union, give rise to compound solids, from which the different organs are formed. A bone, for example, is a compound of various tissues; osseous in its body; medullary in its interior; and cartilaginous at its extremities.

Bichat was the first anatomist who possessed clear views regarding the constituent tissues of the animal frame; and whatever merit may accrue to after anatomists and physiologists, he is entitled to the credit of having pointed out the path, and facilitated the labours of the anatomical analyst.

The term texture can only apply to solids; but inasmuch as there' are in suspension in certain fluids, as the blood, chyle and lymph, solid corpuscles of determinate form and organic properties, and which are not mere products or secretions of a particular organ, or confined to a particular part, such corpuscles have been looked upon as organized constituents of the body, and therefore considered along with the solid tissues; and, accordingly, the textures and other organized constituents have been enumerated as follows:3

'Table Synoptique des Solides Organiques.

2 Anatomie Gén., Paris, 1801, tom. i.

* Quain and Sharpey, Human Anatomy, Amer. edit., by Dr. Leidy, i. 39, Philad., 1849,

Under the idea, now entertained, that all organized tissues are essentially composed of cells having plastic or formative powers, with an intercellular substance or blastema, the tissues have been thus arranged by Schwann,' the great author of the cell doctrine.

1. Isolated, independent cells. To this class the cells in fluids preeminently belong:-lymph globules; blood corpuscles.

2. Independent cells united into continuous tissues; such as the horny tissues and the crystalline lens.

3. Cells in which only the cell walls have coalesced,-cartilage, bone, and the substantia propria (ivory) of the teeth.

4. Fibre cells,-cellular (areolar), fibrous and elastic tissue.

5. Cells in which both the cell walls and cell cavities have coalesced, -muscle, nerve and capillary vessels.

Dr. Allen Thomson' has proposed the following tabular view, which -he remarks-may be adopted in preference to the foregoing as combining similar theoretical considerations, with a more immediate reference to the actual form of the prevailing structural elements in the different tissues. He properly adds, however, that this classification is open-as he might have said every arrangement must be-to several objections; inasmuch as it brings together, under the same head, some parts endowed with different functions; and separates some textures whose functions are closely related; and it does not point out sufficiently the usual degree of complexity of the several textures.

Some part of it, moreover, is founded on theoretical considerations. not yet fully established; and the distinctions on which it rests are based on a structural analysis of various extent in the different textures. On the whole, however, it is a sufficient exponent of the existing state of belief on the subject.

I. Organized textures in which the cellular form of the constituent elements is apparent; not unfrequently also presenting granules of molecular deposition.

1. Rounded simple cells, floating loose in fluid, Blood, Lymph, Chyle and Milk corpuscles, &c.

2. Simple cells massed together, either preserving their cellular form, and without other parts intervening, or altered in form and mixed with

1

Microscopical Researches into the Accordance in the Structure and Growth of Animals and Plants. Sydenham Society's edit., by Henry Smith, p. 66, London, 1847.

2 Outlines of Physiology for the Use of Students, pt. i. p. 68, Edinb., 1848.

other solid elements:-Pigment, Fat, Cuticle, Horny textures, Epithelium, Crystalline lens, Cartilage.

3. Simple cells, or their contents, altered in form :-Ciliated texture, Spermatozoa.

4. Compound cells, separate or mixed with other textures:-Ovum, Ganglionic corpuscles.

II. Textures exhibiting a simply fibrous structure.

1. Filamentous (areolar) texture; formerly Cellular texture.

2. Fibrous textures:-Tendon, Ligament, Fibrous membranes, Fibrous plates.

3. Elastic fibrous texture.

III. Textures exhibiting a tubular structure.

1. Containing moving fluids:-Bloodvessels and Absorbent vessels. 2. Containing muscular substance:-Striated and non-striated muscular fibre.

3. Containing nervous matter:-Primitive nerve tubes.

IV. Textures exhibiting a membranous structure.

1. Principally filamentous:-Serous and Synovial membranes. 2. Filamentous and vascular:-Mucous membranes; True skin. 3. Membrane and cells:-Glands.

4. Membrane and Bloodvessels, &c.:-Lungs.

In combining to form the different structures, the solids are arranged in various ways. Of these, the chief are in filaments or elementary fibres, tissues, organs, apparatuses, and systems. A filament is the elementary solid. A fibre consists of a number of filaments united together. Occasionally, this is called a tissue:—the term tissue usually, however, means a particular arrangement of fibres. An organ is a compound of several tissues. An apparatus is an assemblage of organs, concurring to the same end:-the digestive apparatus consists of the organs of mastication, insalivation, and deglutition, the stomach, duodenum, pancreas, liver, &c. These may be, and are, of very dissimilar character, both as regards their structure and functions; but, if they concur in the same object, they form an apparatus. A system, on the other hand, is an assemblage of organs, all of which possess the same or an analogous structure. Thus, all the muscles of the body have a common structure and function; and form, in the aggregate, the muscular system. All the vessels of the body, and all the nerves, for like reasons, constitute, respectively, the vascular, and nervous systems.

d. Of the Fluids of the Human Body.

The positive quantity or proportion of the fluids in the human body does not admit of appreciation, as it must vary at different periods, and under different circumstances. The younger the animal, the greater is its preponderance. When we first see the embryo, it appears to be almost wholly fluid. As it becomes gradually developed, the proportion of solid parts increases, until the adult age; after which it becomes less and less in the progress of life. During the whole of existence, too, the quantity of fluids in the body fluctuates. At times, there is plethora

or unusual fulness of blood-vessels; at others, the blood is less in quantity.

Experiments have been made for the purpose of ascertaining the relative proportion of fluids to solids. M. Richerand says, that they are in the ratio of six to one; M. Chaussier, of nine to one. The latter professor put a dead body, weighing one hundred and twenty pounds, into a heated oven, and dried it. After desiccation, it was found to be reduced to twelve pounds. It is probable, however, that some of the more solid portions were driven off by the heat employed; and hence, that the estimated proportion of fluids was too high. On this account, M. Bérard1 thinks, that instead of estimating the proportion of liquids at nine-tenths, it would be better to take the mean result of experiments by M. Chevreul, who performed the desiccation in vacuo and with a very moderate heat. This would give the proportion of water in the human body about 6.667 parts in the 10.000.

In the Egyptian mummies, which are completely deprived of fluid, the solids are extremely light, not weighing more than seven pounds; but as we are ignorant of the original weight of the body, we cannot arrive at any approximation. The dead bodies found in the arid sands of Arabia, as well as the dried preparations of the anatomical theatre, afford additional instances of reduction by desiccation. To a less extent, we have the same thing exhibited in the excessive diminution in weight that occurs in disease, and occasionally in those who are apparently in health. Not many years ago, an Anatomie vivante was exhibited in London to the gaze of the curious and scientific, whose weight was not more than eighty pounds. Yet the ordinary functions were carried on, apparently unmodified. In the year 1830, a still more wonderful phenomenon was shown. A man, named Calvin Edson, forty-two years old, five feet two inches high, weighed but sixty pounds. His weight had formerly been one hundred and thirty-five pounds. For sixteen years previously, he had been gradually losing flesh, without any apparent disease, having enjoyed perfect health and appetite, and eating, drinking, and sleeping as well as any one. He was properly called the "living skeleton." It was stated in the public journals2 that Dr. Edson, a brother of Calvin, was to all appearance entirely destitute of flesh. He was, in 1847, forty-two years old; of ordinary height-five feet six inches, and yet weighed only forty-nine pounds. He retained all his faculties apparently in full vigour. We have it also, on the authority of Captain Riley, that after protracted sufferings in Africa, he was reduced from two hundred and forty pounds to below ninety [?].

The fluids are variously contained; sometimes in vessels as the blood and lymph; at others, in cavities-as the fluids secreted by the pleura, peritoneum, arachnoid coat of the brain, &c.: others are in minute areola-as the fluid of the areolar membrane; whilst others, again, are intimately combined with the solids. They differ likewise in density,--some existing in the state of halitus or vapour; others

'Cours de Physiologie, p. 200, Paris, 1848.

2 Philadelphia Public Ledger, Feb. 2, 1847.

3 Narrative of the Loss of the American Brig Commerce, &c., p. 302. New York, 1817.