346 NERVOUS SYSTEM. 



*be seen stretched from one prominence to another, and sepa- 

 rated considerably from the pia mater. It does not dip into 

 the fissures of the brain, but goes directly across them, from 

 the ridge of one convolution to that of the adjacent, so that it 

 is entirely smooth and uniform in its distribution. Notwith- 

 standing the general closeness of its connexion with the pia ma- 

 ter, it may yet be separated from it by careful dissection, by 

 slight maceration, or by thuse of the blow-pipe; dropsical ef- 

 fusions frequently make out the distinction between the two 

 membranes; also the deposite of coagulating lymph. 



Considering this membrane as a single layer, we have to 

 speak of tho dura mater, as lying loosely upon it. But the au- 

 thority of Bichat, sanctioned by the testimony of many other 

 anatomists, has assigned to it a much more considerable extent. 

 For it seems to be well ascertained, both by analogy and by ob- 

 servation, that it is a sac; which, besides covering the external 

 surface of the pia mater, is reflected over the internal surface 

 of the dura mater, and gives to the latter its smooth shining 

 appearance. This lining is on the same principle that the se- 

 rous lamina of the pericardium lines its fibrous lamina, or that 

 the synovial membrane lines the ligamentous attachments- of 

 an articulation. In the early periods of life, it may be sepa- 

 rated from the dura mater, by dissection. Vicq. D'Azyr has 

 related a case in which it was detached by a collection of pus. 

 Its places of reflection to the dura mater are on the basis of the 

 cranium, where the blood vessels and nerves pass into the 

 sheaths formed by the dura mater, and along the blood vessels 

 entering into the sinuses. This membrane is continuous with 

 the tunica arachnoidea of the medulla spinalis. 



The tunica arachnoidea* is considered to pass into the ventri- 

 cles of the brain by the same apertures that the pia mater does, 

 but it is much less manifest there than elsewhere. 



The texture of this membrane is exceedingly delicate and 

 fine. It is always found, in health, in a transparent state, 

 and is furnished with neither red blood vessels nor nerves. 

 It secretes a sort of halitus, or synovia, which facilitates the 



* As the ventricles of the encephalon are but fissures in its structure, this ar- 

 rangement is in oppositiou to that of the fissures on the surface of the encephalon, 

 and in fact, may be doubted from its defective evidence, excepting in the case of 

 the Velum interpositum. 



