600 COMPAEATIVE ANATOMY. 



vesicular enlargements. Finally, in tlie Mammalia sucL. structures 

 do not seem to be developed. 



§ 445. 



Tlie apparatuses tliat produce the lymph-cells are simple in 

 Fishes, where they are placed in the course of the various lymphatics; 

 the cells are produced in the meshes of reticular connective tissue. 

 Where more largely developed, this arrangement gives rise to local 

 enlargements, which accompany the arteries, in consequence of the 

 relation between these vessels and the lymphatics. This arrangement 

 obtains even in the higher Vertebrata, although the cells are not 

 always developed in the sheaths of the arteries. FoUicular enlarge- 

 ments are formed beneath the mucous membrane of the enteric canal, 

 the lymphatics of which are connected with these cell-forming 

 regions. They are either scattered, or variously grouped together 

 (closed glandular follicles). At the commencement of the wall of 

 the enteron these structures form the tonsils already mentioned ; in 

 different parts of the mucous membrane of the mid-gut they are 

 placed closer to one another, and form the so-called Peyerian 

 Glands, which are present in the Reptilia, but are only found to 

 any great extent in the Mammalia. 



When a number of these lymphatic follicles are united together 

 they form larger structures, lymphatic glands, which are also 

 placed on the course of the lymphatic vessels. In Fishes, Amphibia, 

 and Reptilia there are not, so far as we know, any true lymphatic 

 glands. In Birds, also, they seem to be confined to the neck ; it is 

 in the Mammalia only that they are generally present, and in them 

 they are found in other parts of the body, as well as in the chyhferous 

 portion of the lymphatic system of the mesentery. In some 

 Mammals (e.g. Phoca, Canis, Delphinus) the mesenteric glands are 

 united into a single mass, the so-called pancreas Aselli. 



The Spleen is also one of the organs that form lymph-cells; 

 in its histological structure it only differs from the lymphatic glands 

 by the fact that the cells formed in it pass directly into the blood- 

 vessels. Essentially it is formed of a fine lacunar system inter- 

 posed between the efferent and afferent blood-vessels ; this forms 

 the greater part of the so-called parenchyma of the spleen. 



The spleen is found in all Vertebrata save Amphioxus, and is 

 always placed in the region of the stomach, and generally close to 

 the fundus. It forms an elongated or rounded organ of a dark- 

 red colour, which is sometimes, as in various Selachii, broken up 

 into a number of smaller lobules, some of which are, in other cases, 

 converted into secondary spleens. 



§ 446. 



An organ which is very generally present, and which resembles 

 the lymphatic glands in various points of structure, cannot bo 



