600 MANUAL OF HISTOLOGY. 



unbroken envelope. The pia mater is completely closed (Key, Retzius). 

 It appears, moreover, much thinner on the brain than on the cord. 



In it, as is well known, innumerable blood-vessels present themselves, 

 penetrating for the most part into the nervous matter. We have already 

 considered ( 292) this point in speaking of the spinal cord. In the brain 

 their arrangement is analogous. The pia mater, further, is richly supplied 

 with well-developed lymphatic canals. 



Between the pia mater and the brain, or spinal cord, there does not exist 

 any appreciable space. The "epispinal" and "epicerebral" spaces described 

 by His are artificial productions. After personal observation, we do not 

 hesitate for a moment to declare this statement of Key and Retzius as 

 entirely correct. Boll is also of the same opinion. 



We now turn to a very interesting and important point, as bearing 

 upon the lymphatics of the central organs, namely, the nature and rela- 

 tions of the walls of the vessels entering the substance of the brain. 



These are loosely invested in a sheath, which, lying next the tunica 

 media, opens out into the subarachnoid space with a kind of funnel-shaped 

 dilatation. Owing to this arrangement, the sheaths may be artificially 

 injected from the subarachnoid space far into the interior of the brain 

 and spinal cord. Injections, however, which penetrate under the pia 

 mater, or into the actual nervous tissue of brain or cord, are the result of 

 ruptures. There are no such things as " perivascular" spaces, i.e., passages 

 between the adventitial coat and the adjacent neuroglia ; if anything of 

 this kind present itself, it is an artificial production. 



Another very interesting discovery has also been made by Key and 

 Retzius, viz., that nerve-trunks and ganglia are also invested with a 

 similar dural sheath and arachnoid envelope, and which can be artificially 

 filled in the same way. Here, again, then, we have the "subarachnoid" 

 space. 



We glance now finally at the Pacchionian granulations, small round 

 masses of connective-tissue, met with as normal structures principally 

 along the course of the longitudinal sinus. 



Key and Retzius have made some remarkable statements in regard to 

 these. 



They say that if fluid be injected into either the subdural or subarach- 

 noid space, it makes its way easily into the venous sinuses and venous 

 ramifications of the dura mater. The entrance takes place through the 

 spongy tissue of these granulations. We naturally look for further obser- 

 vations on this point. 



The two entrances to the cavities of the brain, the posterior and anterior 



transverse fissures, are closed by the stretch- 

 ing across them of the pia mater (telce cho- 

 roidece). From their inner side, especially 

 in the anterior transverse fissure between 

 the cerebrum and cerebellum, a leaf-like 

 process with large vessels penetrates into 

 the ventricles of the brain, to form there 



*Hr. 563.-E P it.hclial" cells from the *&* pleXU* choridei. This is nothing but a 



human choroid plexus, a, the ceils network of vessels ( 136) embedded in 



from above; ft, the same in profile. homogen eOUS, COlbid, OF, later on, streaked 



connective-tissue containing cells. On its free surface, as far as this 

 exists, we find those peculiar rooted epithelial cells (fig. 563) already 

 described, 87. The ventricles, however, receive no further covering of 



