DISSECTION OF THE DO< ! 207 



Cavum stjbara* iinoidale. — Over the summit of the cerebral convolutions 

 the arachnoid and pia mater form practically one membrane, but in other 

 situations the two membranes are separated by a subarachnoid cavity of vari- 

 able depth, across which passes a felt- work of fine fibres. In some situations, 

 for example at the base of the brain, the subarachnoid cavity is considerable 

 and forms what are known as the subarachnoid cisterns (cisternae subarach- 

 noidales). A cistern of some magnitude exists between the dorsal surface 

 of the medulla and the cerebellum (cisterna cerebellomedullaris). Another 

 lies over the interpeduncular fossa (cisterna interpeduncularis). A third is 

 related to the optic chiasma (cisterna chiasmatis), and is continuous on each 

 side with a small cistern in the Sylvian fissure (cisterna fossae lateralis 

 cerebri [Sylvii]). 



Pia mater encephali. — The thin pia mater of the brain follows all the 

 inecuialities of its surface, and is highly vascular since the smaller branches of 

 the chief cerebral arteries ramify in it preparatory to entering the substance 

 of the brain. Consequently, when the membrane is raised, its deep surface is 

 rough and shaggy from the presence of minute ruptured vessels. 



Dissection.— Cautiously strip the arachnoid from the base of the brain, and 

 follow the arteries thus exposed as far as possible without injury to the 

 brain tissue. 



The brain is supplied with blood from three main sources, namely, the single 

 basilar artery and the two internal carotid arteries. 



Arteria basilaris. — The basilar artery is formed within the vertebral 

 canal by the union of branches of the right and left cerebro-spinal arteries. The 

 single, median vessel, so formed, enters the cranium by the foramen magnum 

 and runs ventral to the medulla and pons to end by dividing into right and 

 left branches slightly aboral to the mammillary body. In its course the basilar 

 artery contributes a number of small branches to the medulla and pons, and 

 about its termination a multitude of fine branches leave it to enter the apertures 

 of the posterior perforated substance between the cerebral peduncles. 



In addition the basilar artery has branches of larger size and specific name. 

 (1) The 'posterior cerebellar arteries (aa. cerebelh posteriores) arise close to the 

 foramen magnum and run, right and left, round the lateral margins of the 

 medulla oblongata to be expended in the cerebellum. (2) The internal auditory 

 arteries (aa. auditivse interna?) are branches either of the basilar or of the 

 posterior cerebellar arteries. Each enters a temporal bone along with the 

 auditory nerve and ends in the tympanum. (3) The anterior cerebellar arteries 

 (aa. cerebelli anteriores) leave the terminal branches of the basilar artery. 

 Bending round the lateral border of the mid-brain, and following the pons, they 

 terminate chiefly over the oral surface of the cerebellum. (4) After the origin 

 of the anterior cerebellar arteries, the two terminal branches of the basilar may 

 be regarded as becoming the posterior cerebral arteries (aa. cerebri posteriores). 



