MEMBRANES OF THE BRAIN. 347 



motions of the brain, and prevents it from adhering. Occa- 

 sionally, this secretion is so much augmented as to constitute 

 a genuine dropsy. 



Of the Pia Mater. 



The Pia Mater, or Tunica Cerebri Vasculosa, is in contact 

 with the substance of the brain. It also is extremely delicate, 

 but, unlike the last, is furnished with an immense number of 

 blood vessels which go to or return from the brain, and are, 

 in most subjects, so abundant that they give a florid appearance, 

 at a little distance, to the whole membrane. Its external face 

 appears entirely smooth, owing to its being covered, and its 

 processes cemented together by the tunica arachnoidea ; but its 

 internal face exhibits these processes as penetrating to the 

 bottom of the fissures of the brain; consequently, it is very un- 

 equal. 



The pia mater presents, along the coarse of the longitudinal 

 sinus, an abundance of those small graniform bodies, existing 

 also in this sinus, and called Glandulte Pacchioni. They beset 

 the veins as they enter into the longitudinal sinus, and even 

 follow them there, so that there is a chain of them from the 

 surface of the pia mater, into the sinus. They are frequently 

 so abundant on the superior part of the hemispheres, near the 

 great fissure, that they cause the dura and pia mater to adhere, 

 as if from inflammation. It is the larger of this kind which 

 frequently produce an absorption of the dura mater, and of the 

 internal table of -the skull. These bodies are also found, along 

 with the pia mater, in the ventricles of the brain, as at the ex- 

 ternal margin of the plexus choroides, around the pineal gland, 

 and at the bottom of the fourth ventricle. 



The Glandulse Pacchioni, wherever found, present a similar 

 appearance and structure, but varying much in size: they are 

 generally in clusters, which repose on common bases. Anato- 

 mists differ much in their opinion concerning them. Bichat 

 acknowledges his complete ignorance on the subject; Portal 

 says that they are only congeries of vessels or of cellular bo- 

 dies filled with fat. Meckel states, that as they are found es- 

 pecially in the latter periods of life, and never before birth, and 

 as they never exist in very great abundance, except in persons 



