232 TWENTY-SECOND LECTURE. 



spinal cord is therefore broken up into numerous smaller 

 spaces. 



The connective-tissue bundles of the arachnoid are invested 

 in a sheath-like manner by the familiar flat stellate endothe- 

 lial cells (Key and Retzius). The latter also fill the connec- 

 tive-tissue spaces, and after treatment with nitrate of silver, 

 show the familiar areolations. 



The connected cavities contain a very watery fluid, the 

 cerebro-spinal fluid. 



The pia mater likewise appears thin and delicate, with 

 similar fiat connective-tissue cells. It is characterized, how- 

 ever, by its immense wealth of blood-vessels ; it is, also, by 

 no means poor in lymphatics. Its numerous nerves are 

 probably desigrred (at least principally) for the vascular 

 walls. 



Our pia mater covers, in close apposition, the nervous 

 masses of the central organ. His, it is true, formerly as- 

 sumed that there was here an epispinal and epicerebral cavity. 

 This does not exist, however ; it is an artefact. More recent 

 observations teach that the blood-vessels entering the ner- 

 vous substance have connective-tissue adventitia, only loosely 

 spread over their so-called tunica media, and that they thus 

 open into the subarachnoidal space with funnel-shaped dila- 

 tations of the outer layers. They may be artificially injected 

 from the subarachnoidal space far into the interior of the 

 brain. 



The nerve trunks and ganglia have, according to Key and 

 Retzius, the same external dural and internal arachnoidal 

 sheath, as well as subarachnoidal spaces. The injection also 

 succeeds here. All this, like the serous sacs, belongs to the 

 lymphatic apparatus. 



The name of the Pacchionian "glands" or granulations 

 has been given to small rounded connective-tissue masses 

 which occur especially at the upper longitudinal venous si- 

 nuses of the brain. 



According to the two frequently mentioned Swedish in- 

 vestigators, the just mentioned structures form transition 



