30 BLOOD 



This attracts fluid from the blood and causes an increase 

 of lymph. The difference in the amount and character 

 of lymph from the abdominal viscera and from the hmbs 

 is explained by assuming that the capillaries of the former 

 are the more permeable. 



In the central nervous system the place of the lymph 

 is taken by the cerebrospinal fluid. It contains a small 

 amount of sugar but is almost free from proteins. 

 Secreted by the choroid plexus into the third ventricle, 

 it passes by the foramen of Majendie in the roof of the 

 fourth ventricle into the subarachnoid space. It passes 

 into the cerebral veins by the Pacchionian bodies. 



THE SPLEEN 



In the splenic pulp the blood-vessels take the form of 

 sinuses, the walls of which are incomplete. The blood, 

 therefore, passes out and mixes with the splenic cells. This 

 is the only situation in the body where the blood comes 

 into direct contact with tissue-cells without the intervention 

 of lymph. 



In the adult spleen, two processes can be seen to take 

 place — destruction of red blood corpuscles and formation 

 of lymphocytes. The first is carried out by large phago- 

 cytic cells, which engulf and digest the red cells. The 

 hgemoglobin is not destroyed in the spleen, since destruction 

 of injected haemoglobin is unaffected by removal of the 

 organ. It is carried by the splenic vein to the hver, where 

 it is converted into bile-pigment. 



The formation of lymphocytes takes place in the Mal- 

 fighian corjmscles, which are masses of lymphoid cells 

 situated around the small arteries and undergoing prolifera- 

 tion. Blood in the splenic vein is said to contain more 

 leucocytes than blood in the splenic artery. 



In foetal life the spleen is said to be one of the seats of 

 formation of red cells. Whether this function is continued 

 after birth is a matter of dispute. Normally, no histological 



