33S GENERAL ANATOMY. 



envelops the cartilages, differs only from the periosteum by 

 being less vascular. It fulfils, with respect to the cartilages, 

 the same functions as the periosteum with reference to the 

 bones, and moreover, it imparts to those which are thin and 

 flexible, a power to resist rupture, and a tenacity which they 

 do not of themselves possess. 



D. OF THE FIBROUS ENVELOPES OF THE NERVOUS SYSTBM. 



524. The nerves have a peculiar envelope, the neurilema, 

 which is of the same nature as the ligamentous tissue. Around 

 the spinal marrow, this envelope loses the firmness of the liga- 

 mentous tissue, and around the brain, where the pise-mater is 

 its continuation, it becomes simply cellular and vascular. The 

 neurilema, much less vascular than the pia3-mater, is still a 

 very vascular part of the ligamentous system. 



525. The dura-mater or menix, which is vascular like the 

 periosteum, differs from this common membrane of the bones, 

 in being lined by the arachnoid, which converts it into a fibro- 

 serous membrane, because it forms a coat or capsule to the 

 brain and spinal marrow, in as much as in the cranium, the 

 only place where it also serves as a periosteum, it contains 

 sinuses or venous canals in its thickness, and forms prolonga- 

 tions or portions between the divisions of the brain. 



E. OF THE COMPOUND FIBROUS MEMBRANES. 



526. The pericardium and perididymes or tunica vagina- 

 lis are, like the dura-mater, fibro-serous membranes, being the 

 result of the intimate union of ligamentous membrane with 

 the external or parietal layer of a serous membrane. 



In the nasal fossae and their sinuses, in the cavity of the 

 tympanum and mastoid cells, at the roof of the mouth and in 

 some other places also, the periosteum is immediately covered 

 by a mucous membrane which is intimately united to it, which 

 constitutes a fibro-mucous membrane. 



These compound membranes resemble, in their texture, 

 functions, and alterations, the two kinds of tissue of which 

 they are formed. 



