96 GENERAL ANATOMY, 



propria, but there are no muscular fibres. The epithelium is continuous with the 

 epidermis and with the delicate internal cuticle, which is all that is present in the 

 epidermic portion of the tube. When the cuticle is carefully removed from the 

 surface of the cutis, these convoluted tubes of epithelium may be drawn out and 

 form short, thread-like processes on its under surface. 



The contents of the smaller sweat-glands are quite fluid ; but in the larger 

 glands the contents are semi-fluid and opaque, and contain a number of colored 

 granules and cells which appear analogous to epithelial cells. 



SEROUS MEMBRANES. 



The serous membranes form shut sacs and may be regarded as lymph-sacs, 

 from which lymphatic vessels arise by stomata or openings between the epithelial 

 cells (see page 86). The sac consists of one portion which is applied to the walls 

 of the cavity which it lines the parietal portion ; and another reflected over the 

 surface of the organ or organs contained in the cavity the visceral portion. 

 Sometimes the sac is arranged quite simply, as the tunica vaginalis testis ; at others 

 with numerous involutions or recesses, as the peritoneum, in which, nevertheless, 

 the membrane can always be traced continuously around the whole circumference. 

 The sac is completely closed, so that no communication exists between the serous 

 cavity and the parts in its neighborhood. An apparent exception exists in the 

 peritoneum of the female ; for the Fallopian tube opens freely into the peritoneal 

 cavity in the dead subject, so that a bristle can be passed from the one into the other. 

 But this communication is closed during life, except at the moment of the passage 

 of the ovum out of the ovary into the tube, as is proved by the fact that no inter- 

 change of fluids ever takes place between the two cavities in dropsy of the perito- 

 neum or in accumulation of fluid in the Fallopian tubes. 1 The serous membrane 

 is often supported by a firm, fibrous layer, as is the case with the pericardium, and 

 such membranes are sometimes spoken of as "fibro-serous." 



The various serous membranes are the peritoneum, lining the cavity of the 

 abdomen ; the two pleurae and the pericardium, lining the lungs and heart respec- 

 tively ; and the tunicae vaginales, surrounding each testicle in the scrotum. 2 

 Serous membranes are thin, transparent, glistening structures, lined on their inner 

 surface by a single layer of polygonal or pavement endothelial cells, supported 

 on a matrix of fibrous connective tissue, with networks of fine elastic fibres, in 

 which is contained numerous capillaries and lymphatics. On the surface of the 

 endothelium between the cells numerous apertures or interruptions are to be seen. 

 Some of these are stomata, surrounded by a ring of cubical epithelium (see Fig. 

 12), and communicating with a lymphatic capillary (see p. 86); others (pseudosto- 

 mata) are mere interruptions in the epithelial layer, and are occupied by pro- 

 cesses of the branched connective-tissue corpuscle of the subjacent tissue or by 

 accumulations of the intercellular cement-substance. 



The secretion of these membranes is, in most cases, only sufficient in quantity 

 to moisten the surface, but not to furnish any appreciable quantity of fluid. When 

 a small quantity can be collected, it appears to resemble in many respects the 

 lymph, and like that fluid coagulates spontaneously ; but when secreted in large 

 quantities, as in dropsy, it is a watery fluid, which gives a precipitate of albumen 

 on boiling. 



SYNOVIAL MEMBRANES. 



Synovial membranes, like serous membranes, are connective-tissue membranes 

 placed between two movable tissues, so as to diminish friction, as between the two 

 articular ends of the bones forming a movable joint ; between a tendon and a 



1 The communication between the uterine cavity and the peritoneal sac is not only apparent in 

 the dead subject, but is an anatomical fact, which is established by the continuity of its epithelium 

 with that covering the uterus, Fallopian tubes, and nmbriic. 



2 The arachnoid membrane, lining the brain and spinal cord was formerly regarded as a serous 

 membrane, but is now no longer classed with them, as it differs from them in structure, and does not 

 form a shut sac as do the other serous membranes. 



