284 LIFE : OUTLINES OF GENERAL BIOLOGY 



the veins, this is not quite true of the spleen. For the splenic arteries 

 end in an open brushwork of capillaries leading into spaces in the 

 pulp, from which another set of open capillaries lead into the 

 splenic veins. Thus it is possible for particles and corpuscles in the 

 pulp to be swept away in the ceaseless ebb and flow. There is no 

 doubt that this once-a-minute compression and dilatation of the 

 spleen involves a considerable amount of internal work, and is of 

 great importance in keeping up a continual passage of blood in 

 and out of this hard-working organ. 



In embryos and very young stages of backboned animals the 

 spleen is one of the places where red blood corpuscles are made. 

 But while this function usually persists throughout life in fishes 

 and tailed amphibians, it seems normally to dwindle away in frogs, 

 reptiles, birds, and mammals. In the last-mentioned the formative 

 function may be in certain cases reawakened ; but this never occurs 

 in birds, though their blood is peculiarly rich in red blood corpuscles. 



But the spleen is not only a cradle, it is a destructor. Many of 

 the worn-out red blood corpuscles are disintegrated in the spleen 

 of birds and mammals; while others seem to be so debilitated in 

 passing through the splenic sponge that it is easy for the liver to 

 give them a coup de grdce when they come to it in the course of 

 their circulatory journey. 



Another function which is usually ascribed to the spleen is the 

 making of white blood corpuscles; but we infer from the recent 

 very careful review by Skramlik that the evidence hardly justifies 

 us in calling this role more than "very probable". Perhaps the 

 white blood corpuscles are detained for a time in the prison of the 

 spleen and are then liberated again in large numbers. 



But this by no means exhausts the role of the spleen. For in 

 most backboned animals, if not in man, the spleen has a share 

 in changing the nitrogenous waste-products and the nitrogenous 

 surplus products into substances like uric acid, which can be readily 

 filtered out by the kidneys. Then again it seems that the spleen 

 shares in the iron-cycle; that is to say, the iron which is set free 

 in the disintegration of the haemoglobin of the red blood corpuscles 

 may be temporarily stored in the spleen and then given up again 

 for re-utilisation. 



Thus while it is true that removal of the spleen is not fatal, 

 because of vicarious functioning on the part of the lymphatic 

 glands, there is no doubt as to the importance and value of the 

 organ, or as to the manifoldness of the r61e that it discharges. It 

 is to be thought of as an active and sensitive organ, thrilling with 

 rapid change to corporeal and external stimuli. Thus when a stickle- 

 back or an eel passes from fresh water to the sea, the weight of the 

 spleen is reduced to about a half in twenty-four hours. There are 

 interesting differences in the spleens of different animals which one 



