CHAPTER II.

SEROUS AND SYNOVIAL FLUIDS-MUCUS-SEBACEOUS FLUIDS.

Physiological anatomy of the serous and synovial membranes-Synovial fringes -Bursa-Synovial sheaths-Pericardial, peritoneal, and pleural secretions-Quantity of the serous secretions-Synovial fluid-Mucus-Mucous membranes—Mucous membranes covered with pavement-epithelium—Mucous membranes covered with columnar epithelium-Mucous membranes covered with mixed epithelium-Mechanism of the secretion of mucus—— Composition and varieties of mucus-Microscopical characters of mucus -Nasal mucus-Bronchial and pulmonary mucus-Mucus secreted by the lining membrane of the alimentary canal-Mucus of the urinary passages -Mucus of the generative passages-Conjunctival mucus-General function of mucus-Non-absorption of certain soluble substances, particularly venoms, by mucous membranes-Sebaceous fluids-Physiological anatomy of the sebaceous, ceruminous, and Meibomian glands-Ordinary sebaceous matter-Smegma of the prepuce and of the labia minora-Vernix caseosa - Cerumen-Meibomian secretion-Function of the Meibomian secretion.

Physiological Anatomy of the Serous and Synovial Membranes.

THE serous and synovial menibranes, which are frequently classed together by anatomists, present several wellmarked points of distinction, both as regards their structure and the products of their secretion. The serous membranes are the arachnoid, pleura, pericardium, peritonæum, and tunica vaginalis testis. The synovial membranes are found around all the movable articulations. They also form elongated sacs enveloping many of the long tendons, and exist in various parts of the body in the form of shut sacs, when they are called bursæ.

Serous Membranes.-The structure of the serous men branes is very simple. They consist of a dense tissue of fibres, which is frequently quite closely adherent to the subjacent parts, and covered by a single layer of pavement, or tesselated epithelium. The fibres are mainly of the inelastic variety arranged in bundles, interlacing each other in the form of a close net-work, and mingled with small, wavy fibres of elastic tissue and numerous blood-vessels. It has not been satisfactorily demonstrated that the serous membranes contain nerves and lymphatics, though the latter are generally quite abundant in the subjacent parts, particularly beneath the visceral layers.' The capillary blood-vessels are in the form of a close, polygonal net-work, with sharp angles.

The epithelium of the serous membranes is pale, regular, with rather large nuclei, and is easily detached after death. Todd and Bowman describe a delicate basement-membrane between the fibrous structure and the layer of epithelium,' but others have not been able to distinguish it, and the existence of such a membrane is considered doubtful.'

These membranes, as a rule, form closed sacs, with their opposing or free surfaces nearly in apposition. The secretion, which is generally very small in quantity, is contained in their cavity. The exceptions to this are the arachnoid membrane, the surfaces of which are exactly in apposition, the fluid being situated beneath both layers, and the peritoneum of the female, which has an opening on either said for the Fallopian tubes.

Synovial Membranes.-The true synovial membranes are found in the diarthrodial, or movable articulations; but in

1 See vol. ii., Absorption, p. 433.

TODD AND BOWMAN, Physiological Anatomy and Physiology of Man, London, 1845, vol. i., p. 130.

3 BRINTON, Serous and Synovial Membranes.—Cyclopædia of Anatomy and Physiology, London, 1847-1849, vol. iv., part i., p. 514.

• MAGENDIE, Mémoire sur un liquide qui se trouve dans le crane et le canal vertébral de l'homme et des animaux mammifères.—Journal de physiologie, Paris, 1825, tome v., p. 36.

various parts of the body are found closed sacs, sheaths, etc., which resemble synovial membranes both in structure and function. Every movable joint is enveloped in a capsule which is closely adherent to the edges of the articulating cartilage and is even reflected upon its surface for a short distance. It was formerly thought that these membranes, like the serous sacs, were closed bags, with one layer attached to the cartilage, and the other passing between the bones so as to enclose the joint; but it is now the general opinion that the cartilage which encrusts the articulating extremities of the bones, though bathed in synovial fluid, is not itself covered by a membrane.

The fibrous portion of the synovial membranes is more dense and resisting and less elastic than the serous membranes. It is composed of white inelastic fibrous tissue, with a few elastic fibres and blood-vessels. The latter are generally not so numerous as in the serous membranes. The internal surface is lined with small cells of flattened, pavement-epithelium, with rather large, rounded nuclei. These cells exist in from one to two or four layers.'

In most of the joints, especially those of large size, as the knee and hip, the synovial membrane is thrown into folds which contain a considerable amount of true adipose tissue. In nearly all the joints, the membrane presents fringed, vascular processes, called sometimes synovial fringes. These are composed of looped vessels of considerable size; and when injected they bear a certain resemblance to the choroid plexus. The edges of these fringes present numerous leaf-like, membranous appendages, of a great variety of curious forms. They are generally situated near the attachment of the membrane to the cartilage. There is no reason for supposing that either the adipose folds or the vascular fringes have any special office in the production of the synovial secretion, different from that of other portions of the membrane, though such a theory has been advanced.

1 KÖLLIKER, Handbuch der Gewebelehre des Menschen, Leipzig. 1867, S. 201.

The arrangement of the synovial bursæ is very simple. Wherever a tendon plays over a bony surface, we find a delicate membrane in the form of an irregularly shaped, closed sac, one layer of which is attached to the tendon, and the other to the bone. These sacs are lined with an epithelium like that found in the synovial cavities, and they secrete a true synovial fluid. Numerous bursæ are also found beneath the skin, especially in parts where the integument moves over bony prominences, as the olecranon, the patella, and the tuberosities of the ischium. These sacs, sometimes called bursæ mucosa, are much more common in man than in the inferior animals, and have essentially the same function as the deep-seated bursæ. The form of both the superficial and deep-seated bursæ is very irregular, and their interior is frequently traversed by small bands of fibrous tissue. The synovial sheaths, or vaginal processes, line the canals in which the long tendons play, particularly the tendons of the flexors and extensors of the fingers and toes. They have essentially the same structure as the bursæ, and present two layers, one of which lines the canal, while the other is reflected over the tendon. The vascular folds, described in connection with the articular synovial membranes, are found in many of the bursa and synovial sheaths.

Pericardial, Peritoneal, and Pleural Secretions.-In the normal condition of the true serous membranes, the amount of secretion is very small; so small, indeed, that it never has been obtained in quantity sufficient for ultimate analysis. It is not true that these membranes produce merely a vaporous exhalation. Their secretion is always liquid, and, small as it is in quantity, it can be found in the pericardial sac, and sometimes in the lower part of the abdominal cavity. As the only apparent function of these fluids is to moisten the membranes, so that the opposing surfaces can move over each other without undue fric

tion, only enough fluid is secreted to keep these surfaces in a proper condition. The error frequently committed by authors, in describing the serous exhalations as vaporous, is due to the fact that a vapor is generally given off when the serous cavities are exposed, either in a living animal or in one recently killed. This vaporous exhalation takes place after exposure of the parts; but if the cavities be observed without exposing the serous surfaces to the air, a certain quantity of liquid can be detected. Colin always found liquid in the peritoneal, pericardial, and pleural cavities of animals recently killed or opened during life. In these cavities the opposite surfaces of the serous membrane were either in contact, or the space between them was filled with liquid. In one of the small ruminants, he removed the muscles and the elastic tunic from the lower part of the abdomen, exposing the transparent peritoneum, and through this membrane could see liquid collected in the dependent parts.'

As far as has been ascertained, the secretions of the dif ferent serous membranes bear a close resemblance to each other. They are either colorless, or of a slight amber tinge, alkaline in reaction, and have a specific gravity of from 1012 to 1020. Their composition resembles that of the serum of the blood, except that the proportion of water is very much greater. They contain albumen, chlorides, carbonate and phosphate of soda, and a little glucose. These facts are the result of observations upon the serous fluids of some of the inferior animals; and it is exceedingly difficult to obtain the normal fluids from the human subject. The elaborate analyses which are sometimes given of the fluids from the different serous cavities in the human subject are the results of examinations of large morbid accumulations.'

1 COLIN, Traité de physiologie comparée des animaux domestiques, Paris, 1856, tome ii., p. 438.

ROBIN, Leçons sur les humeurs, Paris, 1867, p. 262, et seq. This author