224 THE CIRCULATORY SYSTEM. 



2. LYMPH CAPILLARIES, LYMPH-SPACES, AND SEROUS CAVITIES. 



The walls of the lymph capillaries consist of very delicate, flat- 

 tened endothelial cells, which are, however, somewhat larger and 

 more irregular in outline than those of the vascular capillaries. The 

 two may also be further differentiated by the fact that the diameter 

 of the lymph capillaries varies greatly within very short distances. 

 From a morphologic standpoint, the relations of the lymph capil- 

 laries to the vascular capillaries and adjacent tissues are among the 

 most difficult to solve. The distribution of the lymph-vessels and 

 capillaries can be studied only in injected preparations, and it is 

 easily seen that structures of such elasticity and delicacy are pecu- 

 liarly liable to injury by bursting under this method of treatment. 

 The resulting extravasations of the injection-mass then spread out 

 in the direction of least resistance and still further obscure the 

 picture, rendering it difficult to determine what spaces are preformed 

 and what are the result of the injection. So much is, however, cer- 

 tain : that the more carefully and skilfully the injection is made, the 

 greater are the areas obtained, showing the injection of true lymph 

 capillaries. The recent work of W. G. MacCallum confirms this, since 

 he has shown quite conclusively that the lymphatics form a system of 

 channels, with continuous walls, and are thus not in direct commu- 

 nication with the so-called intercellular lymph-spaces the lymph- 

 canalicular spaces. Further confirmation of the fact that the 

 lymphatics form a closed system of channels is found in the excel- 

 lent contribution of Dr. Florence R. Sabin, dealing with the 

 development of the lymphatic system. It is here shown that the 

 lymphatic system begins as two blind ducts, guarded by valves, 

 which bud off from the veins of the neck, and from two similar 

 buds which arise from veins in the inguinal region. These buds 

 grow and enlarge to form lymph-hearts, and from these ducts grow 

 out toward the skin, which they invade and in which they spread 

 out to form anastomosing plexuses. Ducts also grow toward the 

 aorta to form the anlagen for the thoracic ducts, and from these 

 grow out and invade the various organs. 



In some regions very dense networks of lymph capillaries sur- 

 rounding the smaller blood-vessels have been demonstrated. Larger 

 cleft-like spaces, lined with endothelium and communicating with 

 the lymphatic system, are also found surrounding the vessels, peri- 

 vascular spaces. These are present in man in the Haversian canals 

 of bone tissue, around the vessels of the central nervous system, 

 etc., and are separated from the actual vessel-wall by flattened endo- 

 thelial cells. As in the case of the so-called perilymphatic spaces, 

 the walls of the perivascular spaces are joined here and there by 

 connective-tissue trabeculae covered by endothelium. Such struc- 

 tures exist in the perilymphatic spaces of the ear, the subdural 

 spaces of the pia, the subarachnoidal space, the lymph-sinuses, etc. 

 The perivascular spaces are better developed in the lower animals 

 (amphibia, reptilia, etc.) than in mammalia. 



