338 ARACHNOID. 



part to another, as they are severally compressed by sanguineous 

 turgescence during muscular efforts. He says he never observed 

 the fluid escape at the spot near the venae Galeni. There is no 

 doubt that he did not, nor by any other : for large serous accu- 

 mulations often exist in the head, and none in the spine. Most 

 persons have no doubt that the fluid is usually contained, not be- 

 tween the arachnoid and pia mater, but in the serous membrane 

 the arachnoid. Because, 1st, the true anatomy of the parts shows 

 that the old opinion is wrong, shows that the arachnoid is a sac 

 like all other serous membranes, and covers the brain and lines 

 the dura mater ; that the ventricles are lined not with pia mater, 

 but with the arachnoid, under which the pia mater exists only as 

 subserous cellular membrane ; and that hydrocephalus is analo- 

 gous with dropsies of the pericardium, pleura, peritonaeum, and 

 tunica vaginal is. 2dly, If the fluid was in the pia mater, it would 

 be under the arachnoid, and the arachnoid, whether of the surface 

 or of the ventricles, would be raised in proportion to its quantity, 

 so that we should find a membrane upon the surface of the fluid 

 both in the interior and exterior of the brain. This is not the 

 case in common ; and where it is, as in a case described by Dr. 

 Magendie and one by Dr. W. Heberden, presently to be quoted, 

 the close portion of the arachnoid lies conspicuously upon the 

 fluid. 8 1 am therefore satisfied that Dr. Magendie's account is 

 wrong, and that what he calls pia inater in the ventricles is the 

 arachnoid. h 



Dr. Magendie found the removal of the fluid to occasion imme- 

 diate, dulness and immobility; but says that these disappeared as 

 soon as the fluid was replaced, and that its secretion took place very 

 rapidly. He believes that two ounces may exist in the ventricles 

 without disturbance, but that a larger quantity, whether secreted 



8 I may mention that in a child I saw with hydro-rachitis the aqueous tumour 

 in the loins disappeared, and the head immediately enlarged with hydrocephalus. 

 This looked like any thing but communication. 



h He appears to me ignorant of the true anatomy of these membranes, and to 

 confound the two, as was the case of old, till the Anatomical Society of Amster- 

 dam confirmed, in 1665, the doubts which were arising on the subject, and Van 

 Home demonstrated both membranes distinctly to his pupils. 



Ackerman contended that fluid always exists in the ventricles, and for the 

 purpose of maintaining a degree of pressure necessary to the functions of the 

 brain ; an increase or diminution of it arresting the cerebral functions. Sir E. 

 Home repeated the same opinions in the Ph. ^rans. 1814, part ii. See Dr. 

 Spurzheim, Phrenol,, Amer. ed. p. 45. sq. 



