HAEMOLYMPH GLANDS. 159 



HAEMOLYMPH GLANDS. 



Haemolymph glands resemble lymph glands in form and also in size, 

 ranging from that of a "pinhead to an almond." They occur espe- 

 cially in the retro-peritoneal tissue near the origin of the superior mesenteric 

 and renal arteries, but also in the thorax and neck. They are darker than 

 lymph glands, and on section yield blood in place of lymph. No lymphatic 

 vessels are associated with typical haemolymph glands, and instead of a 

 lymph sinus they possess a similar structure filled with blood, the blood 

 sinus. The lymphoid tissue with its blood supply, together w r ith the cap- 

 sule and trabeculae, are like the corresponding structures in lymph glands. 

 The capillary blood vessels, however, are readily permeable so that their 

 contents, both plasma and corpuscles, escape into the blood sinus. The 

 haemolymph gland is therefore a blood filter. Many blood corpuscles 

 fragment and are removed from the circulation by phagocytic cells which 

 in consequence become pigmented. Eosinophilic cells which have been 

 found in haemolymph glands have been explained as due to the ingestion 

 of haemoglobin products. Haemolymph glands have as a second function 

 that of producing lymphocytes which may enter the blood vessels. 



After accidents accompanied by extravasations of blood, the lymph 

 sinuses of lymph glands may be filled with red corpuscles conveyed to them 

 by afferent lymphatics. Such glands should not be confounded with 

 haemolymph glands which have no lymphatic vessels. It has been said, 

 however, that intermediate forms between the two sorts of glands occur, 

 meaning that some normal lymphatic glands contain blood in their sinuses 

 derived from their own blood vessels. The embryology of haemolymph 

 glands is unknown but it is not supposed that they are lymph glands which 

 in the course of development have lost their lymphatic vessels. They are 

 regarded rather as structures which are distinct from the outset, and which 

 are closely related to the spleen. 



SPLEEN. 



The spleen, being five or six inches long and four inches wide, is much 

 the largest organ of the lymph gland series. It is the first of them to de- 

 velop, appearing in rabbits of 14 days (10 mm.) as a condensation of the 

 mesenchyma in the dorsal mesentery of the stomach. At this stage the 

 only lymphatic vessels in the embryo are those near the jugular vein. 

 Lymph glands are not indicated until six days later. The blood vessels 

 enter the spleen at its hilus and branch freely. It is unknown whether or 

 not the artery ever connects with the vein. Surrounding the arterial 

 branches there is a zone of lymphoid tissue which is so highly developed 



