THE MEDULLA OBLONGATA AND BRAIN. 231 



tive-tissue substratum it presents rounded cavities which are 

 sometimes more and sometimes less complete. We here 

 meet with two kinds of cells ; large stellate ones, forming a 

 net-work, and smaller ones. In the adult, the latter have pro- 

 cesses, in the new-born child, however, they were at one time 

 without these (Bizzozero). 



The blood-vessels of the brain, similar to those of the spinal 

 cord, form very compact vascular net-works in the gray sub- 

 stance ; in the white substance the meshes are much wider. 



The arrangement in the individual portions of the brain is 

 often, however, very characteristic and elegant, as for ex- 

 ample in the olfactory lobules, the corpus striatum and the 

 cortex of the cerebellum. We cannot here enter into details. 



We have finally to mention the membranes of the cerebro- 

 spinal system. 



The dura mater (page 57) of the brain is intimately con- 

 nected with the periosteum of the cranial cavity. Around 

 the spinal cord, on the contrary, with the exception of the 

 anterior side, it forms a freely suspended tube. The spaces 

 of the vertebral canal are filled by connective tissue with fat 

 cells. The vascularity is very moderate in the cerebral por- 

 tion, and very slight in the spinal portion. The lymphatics of 

 the dura mater are very abundant. The dura mater of the 

 brain presents nerves of unknown termination. 



The dura mater and arachnoid leave a system of cavities, 

 the subdural space (Key and Retzius), between them. 



The latter membrane, the arachnoid, is very poor in blood- 

 vessels, is thin, delicate and fenestrated in a reticular manner. 



Over the spinal cord it is separated from the lowermost 

 tunic of the pia mater, with the exception of connecting fila- 

 ments of connective tissue. There is thus formed a consider- 

 able subarachnoidal space. Over the brain, on the contrary, 

 the arachnoid and pia mater are for the greater part coalesced 

 with each other, and spaces occur only in those places where 

 the former membrane stretches over the furrows of the sur- 

 face in a bridge-like manner, while the pia mater descends to 

 the bottom. The considerable subarachnoidal space of the 



