48 Th. Lewis, 



in the case of hciemal glands generally, of a fibrous mesliwork, lined 

 throughout by a definite endothelium, ivhich forms a layer more or 

 less continuous ivith the endothelium covering the Malpighian cor- 

 puscles. This arrangement is such that when the blood leaves the 

 capillaries or small arteries entering the spleen pulp, it passes into 

 spaces lined, by endothelium, ivhich together form a large sinus in 

 every luay comparable to that of the typical hœmal glands of the sheep 

 or ox, although in the spleen the reticulum is denser. The endo- 

 thelium of the small arteries and veins entering or leaving the spleen 

 is continuous with the endothelium of the reticulum, though the cells 

 of the latter are to a great extent modified to suit their surroundings. 

 The application of an endothelial cell to an irregular fibrous struc- 

 ture, such as reticulum, must necessarily distort it considerably, and 

 when sections are taken through such an endothelial-lined reticulum, 

 it may often appear to be composed of irregular or branched cells. 

 The blood stream consequently remains in closed spaces, and has little 

 or no contact with adenoid tissue or reticulum, except in the case 

 of the absence of an endothelial cell, which may in many cases be 

 due to its having assumed phagocytic functions. 



As regards the inter-relationships of the different glands under 

 discussion, the general arrangement of the adenoid tissue and sinus 

 forms the first point of consideration. Adenoid tissue exists in each 

 individual of the haemolymph series, from the lymphatic gland to the 

 spleen, but it is more abundant in the former and least in the latter 

 (pi. II. fig. 17 — 20). In the lymphatic glands almost the entire organ 

 is made up of this tissue, broken up considerably by the comparatively 

 small lymphatic sinuses which pervade it (pi. I. fig. 4 and pi. II. 

 fig. 17). In the haemal lymphatic gland (pi. I. fig. 3 and pi. II. fig. 18), 

 there is always a large sinus, usually centrally placed. There are in 

 addition peripheral sinuses connected with the central sinus by numerous 

 smaller blood -lymph spaces, which are extremely subdivided. In the 

 haemal gland of the sheep the peripheral sinus occupies more than 

 half the area of section, and has smaller sinuses in connection with it 

 (pi. I. fig. 2 and pi. II. fig. 19). In the spleen (pi. I. fig. 1 and pi. II. fig. 20), 

 the sinus forms the chief part of the gland; and the Malpighian cor- 



