THE VASCULAR SYSTEM 357 



In both the symmetry is not a primary, but a secondary one; 

 the heart begins and ends with four chambers, it is true, but 

 they do not at all correspond, the latter form resulting from 

 the suppression of two of the first and the subdivision of the 

 remaining two*. <- 



The lymphatic function, that which cares for the blood 

 components which become infiltrated into the tissues, and re- 

 turns them to the general circulation, is performed, not only 

 by spaces and vessels primarily formed for that purpose, but, 

 in the lower forms at least, by the most of the spaces and 

 lacunae of the body that form parts of other systems. In 

 fishes and amphibians a system of lymph channels becomes 

 developed in the loose connective tissue that forms an external 

 sheath for the larger blood-vessels, and here the lymph, not 

 confined within special walls, is intercellular and circulates 

 freely within the meshes formed by the branching connective- 

 tissue cells. One of the largest of these lymph channels is the 

 subvertebral ^space, which enfolds the aorta. The lympatics 

 that collect the digested food (chyle) from the 4 intestine com- 

 municate, either directly or indirectly, with this channel, which 

 thus forms a very primitive thoracic duct. 



A second series of lympliatic^spaces Ts" found immediately 

 beneath the membranes -lining the great serous cavities of the 

 body; such are the sub-peritoneal spaces in the walls of the 

 coelom, the sub-dural and inter-dural spaces connected with 

 the membranes that invest the central nervous system, and the 

 peri- and endo-lymphatic spaces of the inner ear. Still others 



* During fetal life there is, in the mammalian heart, an opening in the 

 interatrial septum, through which the blood of the two atria freely mixes. 

 This is, however, a secondary condition, developed in adaptation to the 

 fetal circulation, as is shown by the fact that the septum is first com- 

 pleted before the opening is formed. In the embryos of the monotremes 

 and marsupials there are several small foramina instead of one big one; 

 which is similar to the condition found in urodeles, where the perforations 

 are also secondary. The large foramen in the mammals, the foramen 

 ovale, persists normally in the human infant for a few days after birth; 

 but is occasionally pemianent, producing the condition known as cyanosis, 

 in which the individual suffers continually from the presence of venous 

 blood in the arterial system. 



