38 HUMAN PHYSIOLOGY. 



are found spaces of irregular shape and size, determined largely by the nature 

 of the organ in which they are found, which have been termed lymph spaces 

 or lacuna, from the fact that during the living condition they are continu- 

 ally receiving the lymph which has escaped from the blood vessels through- 

 out the body. In addition to the connective tissue lymph spaces, various 

 observers have described special lymph spaces in the testicle, kidney, liver, 

 thymus gland, and spleen; in all secreting glands between the basement 

 membrane and blood vessels; around blood vessels (perivascular spaces) 

 and around nerves. The serous cavities 01 the body, peritoneal, pleura], 

 pericardial, etc., may also be regarded as lymph spaces, which are in direct 

 communication by open mouths or stomata with the lymphatic capillaries. 

 This method of communication is not only true of serous membranes, but 

 to some extent also of mucous membranes. The cylindrical sheaths and 

 endothelial cells surrounding the brain, spinal cord and nerves, can also be 

 looked upon as lymph spaces in connection with lymph capillaries. 



The lymphatic capillaries, in which the lymphatic vessels proper take 

 their origin, are arranged in the form of plexuses of quite irregular shape. 

 In most situations they are intimately interwoven with the blood vessels, 

 from which, however, they can be readily distinguished by their larger 

 calibre and irregular expansions. The wall of the lymph capillary is 

 formed by a single layer of epithelioid cells, with sinuous outlines, and 

 which accurately dove'tail with each other. In no instance are valves 

 found. In the villus of the small intestine the beginning of the lacteal is 

 to be regarded as a lymph capillary, generally club-shaped, which at the 

 base of the villus enters a true lymphatic ; at this point a valve is present, 

 which prevents regurgitation. The lymphatic capillaries anastomose freely 

 with each other, and communicate on the one hand with the lymph spaces, 

 and on the other with the lymphatic vessels proper. 



As the shape, size, etc., of both lymph spaces and capillaries are deter- 

 mined largely by the nature of the tissues in which they are contained, it is 

 not always possible to separate the one from the other. Their function, 

 however, may be regarded as similar, viz : the collection of the lymph 

 which has escaped from the blood vessels, and its transmission onward into 

 the regular lymphatic vessels. 



* The blood capillaries not only permit the escape of the liquid nutritive 

 portions of the blood through their delicate walls, but are also engaged in 

 the reabsorption of this transudate as well as in the absorption of hew 

 materials from the alimentary canal. The extensive capillary network 

 which is formed by the ultimate subdivision of the arterioles in the sub- 

 mucous tissue and villi of the small intestine forms an anatomical arrange- 



