NERVOUS SYSTEM. VERTEBRATA. j>] 



D. 728. A horizontal section of the human brain mounted l.y 

 Mr. Greenwood, of Leeds, in glycerine jelly (Miall\ 

 process). 



If this specimen be compared with the section made in 

 a corresponding plane through the left cerebral li,..ni- 

 of a Chimpanzee (D. 072), a striking demonstration j, 

 afforded of the contrast between the human insula (which 

 is wholly covered) and the Anthropoid island of Roil (which 

 is partially exposed on the surface). 



Presented l>y C. G. Wheelhouse, Esq. 

 D. J. Cunningham, Journ. Anat. & Phys., vol. xxxii 

 1898, p. 11. 



Membranes of the Brain. 



The brain is enveloped by at least two distinct sheaths of 

 connective tissue, which serve for its protection and support 

 and give passage to its blood-vessels and lymphatics. The 

 innermost sheath (pia mater, or in lower Vertebrates wln-n 

 two sheaths only are present, the combined rudiment of tin- 

 pia and arachnoid) is closely applied to its surface, and follow- 

 accurately all the brain contours and conveys into its substance 

 blood-vessels and lymphatics. The outermost sheath (dura 

 mater) is a modification of the periosteum, and in some cases 

 (e. g. Mammalia) forms a single sheet with it, but is more 

 generally separated into periosteal and dural layers by a zone of 

 looser tissue. In Teleosts this interdural space is frequently 

 very extensive, and is occupied by a very delicate trabecnlar 

 connective tissue containing a gelatinous substance or fat. Tin- 

 dura mater is separated from the underlying layer or layers by 

 a subdural lymph-space. This in the higher groups is a relatively 

 narrow and almost continuous cavity, but in Elasmobraaohfl and 

 Reptiles is large and is occupied by a delicate trabecular n'-iir. 

 In Birds and Mammals there is interposed between the j.ia and 

 dura mater a third layer (arachnoid) which is nothing but the 

 outer parts of the pia separated off by the increase and conflu> 

 of lymph- spaces. The subarachnoid space thus formed i- 

 traversed by numerous trabeculse and is filled with cerebro-sj.inal 

 fluid. In the Mammalia processes of the arachnoid (glandulic 

 Pacchioni) protrude between the fibres of the dura into the 



VOL. II. 2 J 



