ARACHNOID. 337 



the brain, without following the course of its furrows and promi- 

 nences ;" but it enters the third ventricle by an oval opening, dis- 

 covered by Bichat in the base of the telum choridianum, sur- 

 rounded by the venae Galeni, and leading from a canal called 

 arachnoidean between the corpora quadrigemina and pineal gland, 

 and it lines the third and afterwards the two lateral and fourth 

 ventricles. It is a close sac, thus affording, as the peritonaeum 

 does to the abdominal viscera, a double covering to the whole 

 brain and spinal chord, and to the nerves before their departure 

 through the foramina of the dura mater, and lining the four ven- 

 tricles ; insulating the organs on which it lies, and affording them 

 great facility of movement ; and liable to all the morbid affections 

 of serous membranes. 



Between the pia mater and arachnoid of both the brain and 

 spinal chord, Dr. Magendie says he has discovered the existence 

 during life, of a large quantity of clear and colourless fluid, pass- 

 ing from the surface of one organ to that of the other. d Cotugno e 

 had long ago asserted its existence in the cranial and spinal 

 cavities, after death, and its free communication, and accurately 

 described its qualities ; but, notwithstanding he gave excellent 

 reasons for believing its existence during life, he imagined the 

 space around the spinal chord, observed by him to be larger in 

 the emaciated and old, and the space which in these two descrip- 

 tions of subjects he found also around the brain, to be filled with 

 an aqueous vapour ; he also believed its occasional mixture with 

 the fluid of the ventricles. Many deny at present that more than 

 vapour exists during life and health in any serous membrane. But 

 I certainly saw, as I formerly mentioned, a large quantity of clear 

 fluid pass from the spinal canal the instant that Dr. Magendie 

 opened it, in one of his barbarous experiments, which, I am 

 ashamed to say, I witnessed, and in which he began by coolly 

 cutting out a large round piece from the back of a beautiful little 

 puppy, as he would from an apple dumpling. Dr. Magendie 

 thinks he has proved the communication, not only of the fluids 

 of the spinal with that of the cranial cavity, but also of these with 

 that of the ventricles, by an opening at the point of the calamus 

 scriptorius of the fourth. f He conceives it to move from one 



c Bichat, Traite des Membranes. 

 d Journal de Physiologie, t. v. 



e Dissertatio de Ischiade Nervosa. Published in Sandefort's Thesaurus, 

 { Journal de Physiologie, t. vii. p. 21, 

 A A 2 



