PHYSIOLOGICAL ANATOMY OF THE SYNOVIAL MEMBRANES. 351 



the fluid being situated beneath both layers. The peritoneum of the female has an open- 

 ing on either side for the Fallopian tubes. 



Synovial Membranes. The true synovial membranes are found in the diarthrodial, or 

 movable articulations ; but in various parts of the body are found closed sacs, sheaths, 

 etc., which resemble synovial membranes both in structure and in function. Every mova- 

 ble joint is enveloped in a capsule, which is closely adherent to the edges of the articu- 

 lating cartilage and is even reflected upon its surface for a short distance. It was for- 

 merly 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 incrusts the articu- 

 lating extremities of the bones, thougli 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 pave- 

 ment-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 the hip, the syno- 

 vial 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 mem- 

 brane 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, although such a theory has been 

 advanced. 



The arrangement of the synovial barsse 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 bursas are also found beneath the skin, espe- 

 cially in parts where the integument moves over bony prominences, as the olecranon, 

 the patella, and the tuberosities of the ischium. These sacs, sometimes called bursa3 

 mucosa3, are much more common in man than in the inferior animals and have essen- 

 tially the same function as the deep-seated bursas. The form of both the superficial and 

 deep-seated bursaa 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 bursae, and present 

 two layers, one of which lines r 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 bursa3 and the synovial sheaths. 



Pericardia^ 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 



