190 



THE ORGANS 



structure, (i) A protoplasmic endothelial syncytium, with scattered oval 

 nuclei (their long diameter lying in the long diameter of the vessel) and no 

 evidence of cell boundaries. This syncytium is arranged as a network, the 

 meshes of which are sometimes quite irregular, at other times quite regular and 

 rectangular, with the long diameter of the mesh running in the long diameter of 

 the vessel. This protoplasmic syncytium lines the sinus. (2) Outside the 

 endothelial syncytium are closely-placed longitudinal fibres w^hich lie upon the 

 lengthwise-running strands of the protoplasmic reticulum (Fig. 113). (3) Out- 

 side the lengthwise-running strands transverse fibres or ring fibres which at 

 rather longer intervals encircle the tube (Fig. 114). Both longitudinal and ring 

 fibres are connective tissue (reticular), and the reticulum which they form is 

 everywhere continuous with the reticulum of the pulp cords and apparently 

 identical with it, except that to form walls, the reticulum is flattened out and 

 usually has a more regular arrangement. The strands of the fibre reticulum lie 

 upon the strands of the protoplasmic reticulum in such a way that the meshes 

 correspond. A thin structureless fenestrated membrane lying just outside the 

 endothelial syncytium has been described. The sinus walls probably possess a 

 certain elasticity or contractility and undoubtedly change their diameter to 

 accord with varying functional conditions. It follows that the meshes of the 



reticulum may be at one time widely open (con- 

 gested spleen), at another partly open, at still 

 another entirely closed. On the other hand 

 the already noted (p. 188) remarkably uniform 

 diameter of the sheathed arteries probably tends 

 to prevent any sudden flooding of the sinuses 

 and parenchyma. 



The main functions of the adult spleen in 

 health appear to be the production of leuco- 

 cytes and the destruction of red blood cells. It 

 is possible that to a limited degree the spleen 

 produces red blood cells. During foetal life the 

 spleen is actively engaged in the development of 

 both red and white cells. Also in adult life a 

 severe secondary anaemia or a pernicious anaemia 

 may stimulate the spleen to resume production 

 of red and white cells. The spleen can be re- 

 moved without apparently seriously interfering 

 with the body functions. Enlargement of 

 lymph nodes and increased blood-forming 

 activity of bone marrow as a result, have been 

 described. 



The ultimate origin of the spleen in man has 



not been definitely determined. The anlage of 



the spleen can be seen in embryos of five weeks 



as a thickening of the mesenchyme in the left dorsal mesogastrium and it was 



believed that the organ developed wholly from this mesenchyme. The meso- 



thelium covering the thickened mesenchyme also shows thickening, and 



%^ ^ sai* |4| 



Fig. 1 14. — Diagram of a Sinus 

 of the Splenic Pulp, showing Re- 

 lations of Longitudinal Fibres, 

 Circular Fibres and Endothelial 

 Nuclei. (AloUier.) 



