298 MICROSCOPIC ANATOMY OF THE ORGANS. 



arachnoidal space. The inner layer (intinia }>ia) is a thin 

 membrane made up of connective-tissue bundles running cir- 

 cularly. 



The pia mater of the brain resembles the intima pia of 

 the spinal cord. 



The pia mater contains many vessels, partly belonging to 

 itself (plexus chorioideus) and partly derived from those which 

 enter the brain and cord. The vessels of the pia mater spinal is 

 run between the two layers. 



Numerous fine nerve branches, partly from the sympathetic 

 system and partly from cerebro-spinal nerves, enter the pia 

 mater, to supply the vessel walls. They pass into the cord and 

 brain together with the vessels. 



The telce chorioidece and plexus chorioidei are structures 

 which are formed from the pia mater and ventricular epi- 

 thelium. They consist of a connective-tissue membrane which 

 contains many blood-vessels, and is covered by a layer of cub- 

 ical epithelium. The layer represents the much-thinned brain 

 wall, and in the embryo consists of ciliated cells. The plexuses 

 contain no nerves. 



F. Blood-vessels of the Central Nervous System. 



The blood-vessels of the spinal cord have the following 

 relation, according to the work of H. Kadyi : The arterial 

 stems running along the nerve roots (arteries of the ventral 

 and dorsal roots) branch many times in the pia mater and 

 join with one another by numerous anastomoses. There can 

 be distinguished nine longitudinally disposed lines of anasto- 

 mosis, of which the ventral unpaired one is the greatest and is 

 in connection with the arteria spinalis ventralis. From this a 

 series of arterial branches proceed with the pia into the fissura 

 mediana ventralis, giving off side branches right and left into 

 the column of gray matter (central arteries). From other parts 

 of the arterial network of the pia mater numerous fine branches 

 enter the white matter (peripheral arteries), and extend as far as 

 the gray matter. The central and peripheral arteries do not anas- 

 tomose with one another, but are what Cohnheim termed end- 



