LECTURE XIII. 



the deeper parts, for even with the naked eye, and espe- 

 cially after the addition of acetic acid, an external grey 

 and translucent layer is very distinctly seen, whilst the 

 deeper layer looks white. This white appearance is due 

 to the presence of medullated nerve-fibres which first 

 occur singly, then continually become more numerous 

 and closely aggregated, and as a rule run parallel to the 

 surface. Thus it may certainly appear as if a particular 

 membrane existed here, which could be separated from 

 the uppermost nerve-fibres. But now, if we compare 

 the substance which advances to the surface with that 

 which lies between the nerve-fibres, no essential diffe- 

 rence presents itself ; on the contrary, it turns out that 

 the superficial layer is nothing more than an extension 

 upwards, beyond the nervous elements, of a portion of 

 the interstitial tissue which is everywhere present be- 

 tween them, but in this layer alone is seen in all its 

 purity. The connection therefore is a continuous one. 



You see from this description that it was a very idle 

 dispute, when it was discussed for years, whether the 

 membrane which clothed the ventricles was a continua- 

 tion of the arachnoid or pia mater, or was a special mem- 

 brane. There is, strictly speaking, no membrane at all 

 present, but it is the surface of the brain itself which 

 directly meets the eye. In the case of articular cartilage 

 also we must call it idle to dispute what kind of mem- 

 brane invests the cartilage, since the cartilage itself ad- 

 vances right up to the free surface of the joint. Neither 

 is there any prolongation from the arachnoid or the pia 

 mater to the surface of the ventricle ; the last processes 

 which these membranes send inwardly are the choroid 

 plexuses and the tela chorioides [velum interpositum] . 

 Beyond these there is no serous covering found investing 

 the internal surface of the ventricles of the brain. For 

 this reason the conditions of the cerebral cavities cannot 



