CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 487 



somewhat peculiar relations of the lymphatics in the central 

 nervous system we shall consider when we come to treat of that 

 system. Meanwhile we have said enough to form a general idea 

 of the arrangements by means of which the very elements of all 

 the tissues are bathed with lymph, and by means of which that 

 lymph is carried back from the elements of the tissues along 

 irregular and regular lymphatic channels back to the blood from 

 whence it originally came. 



289. The Serous Cavities. In the mammal lymph-spaces are 

 for the most part minute and microscopic; but in some other 

 animals they may attain considerable size; in the frog for instance 

 in which lymph-capillaries and lymphatic vessels are scanty, the 

 large subcutaneous spaces which are disclosed when the skin of the 

 back is cut through are in reality lymph-spaces lined by sinuous 

 epithelioid plates. Both in the mammal and other animals certain 

 large cavities, known as serous cavities, such as the peritoneal, 

 pericardial, pleural and other cavities, must be considered as parts 

 of the general lymphatic system, and indeed the 'serous fluid' 

 which they contain is in reality lymph. The subarachnoid space 

 surrounding the brain and spinal cord may also perhaps be regarded 

 as a part of the lymphatic system, but this and the contained 

 cerebro-spinal fluid we shall consider in connection with the central 

 nervous system. 



In the abdomen of the frog, on each side of the vertebral 

 column, behind or above, i.e. dorsal to the peritoneal cavity, lies a 

 large lymph-space spoken of as the cisterna magna lymphatica, the 

 cavity of which is separated from the peritoneal cavity by a thin 

 membranous sheet consisting of a median basis of connective-tissue 

 covered on the peritoneal side by peritoneal epithelium and on the 

 cisterna side by lymphatic epithelium. The latter consists, as in a 

 lymphatic capillary, of flat epithelioid plates with sinuous outlines ; 

 the former is made up also of flat epithelioid plates but these are 

 more or less polygonal in shape and have outlines which are not 

 distinctly sinuous. If a piece of this partition, after being stained 

 with silver nitrate, be spread out and examined either with the 

 peritoneal or with the cisterna side uppermost, it will be seen that 

 in each case here and there a group of cells assuming a triangular 

 form appear to converge to or radiate from a centre which some- 

 times, especially on the lymphatic side, is a mere point but some- 

 times is a larger or smaller hole, which in other words is an orifice 

 or stoma, sometimes closed but sometimes more or less open. On 

 the peritoneal surface the stoma is surrounded and guarded by a 

 crown of what appear to be small granular cells placed at the 

 apices of the converging epithelioid plates, but which are held by 

 some to be the displaced nuclei of the epithelioid plates themselves. 

 Around each stoma which is in reality a perforation leading from 

 the peritoneal cavity into the cisterna, the connective-tissue basis 

 between the two epithelioid layers is arranged in a concentric 



