340 MOTION OF THE BRAIN. 



lobe and the upper surface of the mesocephalon. These three 

 fissures, united, and establishing a communication between the ex- 

 ternal and internal parts of the brain, were named the great cere- 

 bral fissure by Bichat. It runs over the third ventricle, forming 

 with the arachnoid what is called the velum interposition or telum 

 choridianum ; and the sides of this portion, extended and filled 

 with a plexus of vessels, form with them, in each lateral ventricle, 

 what is called the plexus choroides, also, of course, covered by the 

 arachnoid. 



The pia mater invests the spinal chord equally with the ence- 

 phalon, but is there paler and firmer. It also invests all the 

 nerves, and not only their chords and fibres, but their individual 

 fibrils and filaments. 



Dr. Macartney finds the pia mater to consist of two portions, 

 one of which is exceedingly subtle and pervades the whole 

 encephalic mass, acting as a framework for the nervous substance. 

 Its delicacy allows the external portion to be readily separated 

 from it on the surface ; and it forms, he says, so large a portion 

 of the mass, that the amount of nervous substance, as was re- 

 marked above, is very small. 11 



If from deficiency of cranium the brain is seen, it is observed 

 to experience two motions the one correspondent with the im- 

 pulse of blood into the arteries, the other correspondent with the 

 distension of the veins by expiration. It slightly pulsates at the 

 stroke of the left ventricle ; rising during expiration, and sinking 

 again during inspiration ; and it sinks in proportion as inspiration 

 is desisted from the longer. 



It is found also in such cases to be more distended during the 

 waking state than during sleep P: a circumstance showing that 



n Report of the Third Meeting of the Brit. Scient. Assoc., p. 454. 



" T. Dan. Schlichting first accurately described this striking phenomenon. 

 Commerc. litter. Noric. 1744. p. 409. sq., and more largely, Mgm. presenttes a 

 VAcad. des Sc. de Paris, t. i. p. 1 1 3. 



Haller sagaciously discovered the cause of it by numerous dissections of living 

 animals. J. Dit. Walstorf, his pupil, Experimenta circa motum cerebri, cerebelli, 

 &c. Getting. 1753. 



Consult also, after F. de la Mure's works, Lorry's dissertations on the same 

 point, Mem. presentees, t. iii. p. 277. sq. 344. sq. 



Also Portal on a similar motion observable in the spinal chord, Me"m. sur la 

 Nature de plusieurs Maladies, t. ii. p. 81." 



p " I once enjoyed an opportunity of very distinctly observing this motion, and 

 making some experiments with respect to it, in a young man eighteen years old. 



