442 THE BRAIN 



extends into the longitudinal fissure with the anterior cerebral arteries. 

 A dilatation of the subarachnoid space over the dorsum of the mid-brain 

 round the great cerebral vein (O.T. vena magna Galeni) is called the 

 ci sterna vena magna: cerebri. 



Granulationes Arachnoideales (O.T. Pacchionian Bodies). 



The connection of the arachnoideal granulations with the 

 arachnoid has been referred to already (p. 202). 



Pia Mater Encephali. The pi a mater forms the immediate 

 investment of the brain. It is finer and more delicate than 

 the corresponding membrane of the spinal medulla, and it 

 follows closely all the inequalities on the surface of the brain. 

 Thus, in the case of the cerebrum, it lines both sides of every 

 sulcus and forms a fold within it. On the cerebellum the 

 relation is not so intimate ; it is only the larger fissures of 

 the cerebellum which contain folds of pia mater. 



It has been noted that the larger blood vessels of the 

 brain run in the subarachnoid space ; the finer twigs enter the 

 pia mater and ramify in it before passing into the substance 

 of the brain. As they enter they carry with them sheaths 

 derived from the pia mater. Consequently, if the dissector 

 raises a portion of this membrane from the surface of the 

 cerebrum, a number of fine processes will be seen to be with- 

 drawn from the cerebral substance. These are the blood- 

 vessels, and they give the deep surface of the membrane a 

 rough and flocculent appearance. 



The pia mater is not confined to the exterior of the brain. 

 A fold is carried into its interior. This will be exposed in 

 the dissection of the brain, and will be described under the 

 name of the tela chorioidea (O.T. velum interpositum) of the 

 third ventricle. 



Dissection. The blood vessels of the brain should now be followed out 

 as far as it is possible to do so without laceration of the brain substance. 

 Begin by stripping the arachnoid from the base of the brain. This will 

 bring into view the main trunks. As the vessels of the brain are described, 

 many parts which have not yet come under the notice of the dissector must 

 be mentioned. 



Arteries which supply Blood to the Brain. Four main 

 arterial trunks carry blood into the cranium for the supply 

 of the brain viz., the two internal carotid arteries and the 

 two vertebral arteries. The vertebral arteries enter through 

 the foramen magnum, whilst the internal carotid arteries gain 

 admittance through the lacerate foramina after traversing the 

 carotid canals. These vessels have been divided in the 



