PIA MATER OP THE BRAIN. 399 



the ventricles is beneath the posterior lobes of the cere- 

 brum, in front of the anterior edge of the tentorium, and 

 below the veins of Galen, where will be noticed a small 

 foramen or canal, leading forward above the pineal gland, 

 and opening into the third ventricle. The arachnoid is 

 more loosely attached at some points than others, leaving 

 spaces, which are named according to their locations, as 

 the anterior, posterior, lateral, and superior sub-arachnoid 

 spaces. 



Function. To secrete a fluid serum which lubricates its 

 surfaces, and prevents friction. When this fluid is in 

 excess in the arachnoid cavity, that is, between its parietal 

 and visceral portions, it constitutes that variety of dropsy 

 known by the name of hydrocephalus externus ; if the ex- 

 cess be in the ventricles, it is called hydrocephalus internus. 

 It gives also a sheath to the veins as they enter the supe- 

 rior longitudinal sinus, and a covering to the nerves as 

 they leave the brain. 



The pia mater forms the third and innermost membrane 

 of the brain. It differs from the spinal, with which it is 

 continuous in being vascular instead of fibrous. It is im- 

 mediately in apposition with the brain, and is much more 

 extensive than the arachnoid ; for it not only covers the 

 whole of the external surface of all the convolutions, but 

 dips down between, covering, every where, the external 

 gray matter, and finally entering the ventricles, and there 

 forming folds, such as the choroid plexuses, &c. 



It can be drawn out from between the various convolu- 

 tions and expanded, when it presents the appearance of an 

 extremely delicate membrane loaded with vessels; hence 

 its structure is regarded as essentially vascular. It is the 

 nutritious membrane of the brain, the capillary arteries 

 passing from it into the brain, and the veins going from it 

 into the sinuses. Its external surface is in relation with 

 the arachnoid, to which it is closely and inseparably at- 

 tached upon the surface of the convolutions, while at the 

 base of the brain, and in the different sulci, between the sev- 

 eral convolutions, they are distinct and readily separated. 



