664 LYMPHATIC SYSTEM. 



Lymphatic System. 



The lymphatic system arises from spaces in the general parenchyma of 

 the body, independent in their origin of the true body cavity, though com- 

 municating both with this cavity and with the vascular system. 



In all the true Vertebrata certain parts of the system form definite trunks 

 communicating with the venous system ; and in the higher types the walls of 

 the main lymphatic trunks become quite distinct. 



But little is known with reference to the ontogeny of the lymphatic vessels, 

 but they originate late in larval life, and have at first the form of simple 

 intercellular spaces. 



The lymphatic glands appear to originate from lymphatic plexuses, the 

 cells of which produce lymph corpuscles. It is only in Birds and Mammals, 

 and especially in the latter, that the lymphatic glands form definite struc- 

 tures. 



The Spleen. The spleen, from its structure, must be classed with the 

 lymphatic glands, though it has definite relations to the vascular system. 

 It is developed in the mesoblast of the mesogastrium, usually about the 

 same time and in close connection with the pancreas. 



According to Miiller and Peremeschko the mass of mesoblast which 

 forms the spleen becomes early separated by a groove on the one side from 

 the pancreas and on the other from the mesentery. Some of its cells 

 become elongated, and send out processes which uniting with like processes 

 from other cells form the trabecular system. From the remainder of the 

 tissue are derived the cells of the spleen pulp, which frequently contain more 

 than one nucleus. Especial accumulations of these cells take place at a 

 later period to form the so-called Malpighian corpuscles of the spleen. 



BIBLIOGRAPHY of Spleen. 



(501) W. Miiller. "The Spleen." Strieker's Histology. 



(502) Peremeschko. " Ueb. d. Entwick. d. Milz." Sitz. d. Wuti. Akad. 

 Wiss., Vol. LVI. 1867. 



Suprarenal ^bodies. 



In Elasmobranch Fishes two distinct sets of structures are found, both of 

 which have been called suprarenal bodies. As shewn in the sequel both of 

 these structures probably unite in the higher types to form the suprarenal 

 bodies. 



One of them consists of a series of paired bodies, situated on the 

 branches of the dorsal aorta, segmentally arranged, and forming a chain 

 extending from close behind the heart to the hinder end of the body cavity. 

 Each body is formed of a series of lobes, and exhibits a well-marked 

 distinction into a cortical layer of columnar cells, and a medullary substance 

 formed of irregular polygonal cells. As first shewn by Leydig, they are 



