388 LYMPHATIC SYSTEM. 



In mucous membranes, especially that lining the alimentary canal, conspicuous 

 lymphatic nodules are met with in various parts, and here they have been long 

 known. They occur either singly, as in the so-called solitary glands of the intestine, 

 or collected into groups as in the agminated glands or patches of Peyer, or into 

 thick masses as in the tonsils. In most of these cases the nodules are spherical or 

 dome-shaped condensations of the lymphoid tissue which occurs in the substance of 

 the mucous membrane, on the surface of which they may cause a distinct pro- 

 minence ; they are usually found to be in close relation with the lymphatics of the 

 membrane, being either partially surrounded by a large sinus-like lymphatic, or 

 encircled by a plexus of lymphatic vessels. In the mucous membrane of the 

 bronchial tubes lymphoid nodules are met with which are quite similar to those of 

 the alimentary mucous membrane. In the spleen, tracts of lymphoid tissue, with 

 lymphatics in connexion with them, ensheath the smaller arteries and are dilated 

 at certain points into distinct nodules which have here been long known as the 

 Malpighian corpuscles of the spleen. Lastly the thymus gland seems to be chiefly 

 composed in the young subject of lymphoid tissue having a more or less nodulated 

 arrangement, although in the adult this organ is usually found to have become 

 transformed into adipose tissue. 



It has been shown by Flemming that the nodular formations of lymphoid tissue 

 are in all cases due to the rapid multiplication of cells which is occurring at the 

 spots where they are found ; that they represent, in short, foci of multiplication. 



The further description of the lymphoid structures will be deferred until the 

 several organs where they occur are systematically treated of. 



SEROUS MEMBRANES. 



The serous membranes are so named from the apparent nature of the flnid with 

 which their surface is moistened. They lie in cavities of the body which have no 

 obvious outlet, and the chief examples of them are, the peritoneum^ the largest of 

 all, lining the cavity of the abdomen ; the two pleura and the pericardium in 

 the chest ; and the tunica vaginalis surrounding each of the testicles within the 

 scrotum. 



The arachnoid membrane, which is a delicate connective tissue membrane surrounding the 

 brain and spinal marrow in the bony cavities in which they are contained, was formerly 

 reckoned amongst the serous membranes ; but neither in the details of its structure, in its 

 general disposition, nor in its development does it correspond with the other serous mem- 

 branes. It is, therefore, no longer classed with them, but will be described with the other 

 membranes investing the brain and spinal cord. 



Form and arrangement. In all cases a serous membrane has the form of a 

 closed sac, one part of which is applied to the walls of the cavity which it lines, the 

 parietal portion ; whilst the other is reflected over the surface of the organ or organs 

 contained in the cavity, and is therefore named the reflected or visceral portion of 

 the membrane. Hence the viscera in such cavities are not contained within the sac 

 of the serous membrane, but are really placed behind or outside of it ; seeming to 

 push inwards the part of the membrane which immediately covers them, some 

 organs receiving in this way a complete, and others only a partial and sometimes 

 very scanty investment. 



In passing from one part to another, the membrane frequently forms folds which 

 in general receive the appellation of ligaments, as, for example, the folds of 

 peritoneum passing between the liver and the parietes of the abdomen, but which 

 are sometimes designated by special names, as in the instances of the mesentery, 

 mesocolon. and omentum. 



The peritoneum in the female sex, is an exception to the rule that serous mem- 



