PIA MATER. 339 



or injected, for example, into the spinal cavity, causes more or 

 less apoplexy and palsy. Much must, however, depend upon the 

 quickness of the accumulation, as the powers of accommodation 

 are very great in living systems, and if the bones expand, very 

 many pints may exist without impediment to the functions of the 

 brain. Dr. W. Heberden knew a man who had been long deaf 

 only, with the exception of vertigo and a temporary attack 

 of confusion, and who suddenly died ; when not less than eight 

 ounces of fluid was found in the ventricles, and four on the brain 

 under the arachnoid. * Morgagni gives a similar case. k Still these 

 might have been instances of rapid effusion. 



The blood-vessels of the brain are the two internal carotids 

 and the two vertebrals. They are twice bent at their entrance 

 into the cranium, to lessen the force of the blood ; for not only is 

 the organ delicate, but its arteries are thinner and weaker than 

 others of the same size. In some brutes the internal carotid 

 splits, for the same purpose, into a network of vessels, called rete 

 mirabile, which re-unite into a trunk. The veins of the brain 

 pour their blood obliquely into strong, winding sinuses, which 

 transmit it to the internal jugulars (Cut, p. 336.) ; and the pos- 

 sibility of its reflux into the cerebral veins is thus lessened. The 

 cerebral arteries are said by Beclard to have no third, external 

 coat. 



The pulpy substance has an immense number of blood vessels ; 

 the fibrous, a smaller number. M. Raspail exhibits one blood- 

 vessel in the membranous investment of the median nerve, and 

 many in the coverings and septa of the ganglion of the sympa- 

 thetic, but none appeared to enter into the fibres. (Cut 2, p. 326.) 



" The membrane, called pia mater by the ancients," corresponds 

 with cellular membrane, tc closely follows the cortical substance of 

 the brain 1 , and possesses innumerable blood-vessels which pene- 

 trate into the latter. Hence, if a portion of this membrane is 

 detached, we find the external surface very smooth, while the 

 internal is villous and resembles the roots of moss." m . It pene- 

 trates into the lateral ventricles at the semicircular fissure which 

 exists on each side between the corpus fimbriatum of the fornix 

 and thalamus opticus ; and into the third ventricle, at the central 

 fissure which exists between the posterior extremity of the meso- 



1 Trans, of the Coll. of Phi/., vol. v. k Ep. Anat., 4. 35. 



1 " Ruysch, Respons. ad ep. problemat. nonam. Amst. 1670. tab. x." 

 m " B. S. Albinus, Annot. Acad. 1. i. tab. ii. fig. 1 5." 

 A A 3 



