ORIGIN OF CRANIAL NERVES. 177 



artery. And behind is -the bifurcation of the basilar trunk into the poste- 

 rior cerebrals. In the area^of the circle lie several parts corresponding 

 with the floor_of the third ventricle. 



The complete inosculation between the cranial vessels in the circle of 

 Willis allows at all times a free circulation of blood through the brain, 

 even though a large vessel on one side of the neck should be obstructed. 



The VEINS of the brain enter the sinuses of the dura mater, instead of 

 uniting trunks as companions to the arteries. 



Two sets of veins belong to the cerebrum, viz., superficial or external, 

 and deep or internal. 



The external veins of the upper surface are collected into the superior 

 longitudinal sinus (p. 29) ; and those of the lateral and under parts enter 

 the sinuses in the base of the skull, especially the lateral sinus. 



The deep veins of the interior of the cerebrum join the veins of Galea, 

 and reach the straight sinus. 



The veins of the cerebellum end differently above and below. On the 

 upper surface they are received by the veins of Galen and the straight 

 sinus; and on the lower surface they terminate in the occipital and lateral 

 sinuses. 



Dissection. The pia mater and the vessels are now to be stripped from 

 the brain, and the origin of the cranial nerves is to be defined. Over the 

 greater part of the cerebrum, the pons, and the medulla, the pia mater can 

 be detached with tolerable facility by using two pairs of forceps ; but on 

 the cerebellum the membrane adheres so closely as to require some care in 

 removing it without tearing the substance of the brain. 



In clearing out the fissure between the halves of the cerebellum on the 

 under surface, the membrane bounding the opening of the fourth ventricle 

 will probably be taken away : therefore the position, size, and limits of 

 that opening between the back of the medulla oblongata and the cerebel- 

 lum should be now noted. 



When the surface has been cleaned, the brain is to be replaced in the 

 spirit, but it is to be turned over occasionally, so that all the parts may be 

 hardened. 



SECTION II. 



ORIGIN OF THE CRANIAL NERVES. 



TifE cranial nerves take origin from the encephalon, with one exception 

 (spinal accessory), and pass from it through apertures in the skull. 



The origin of a nerve is not determined by the place at which it appears 

 on the surface of the brain, for fibres or roots may be traced deeply into 

 the brain substance. Each nerve has therefore a superficial or apparent, 

 and a deep or real origin in the encephalon. 



With respect to the superficial attachment there cannot be any doubt ; 

 but there is much difference of opinion concerning the deep origin, in con- 

 sequence of the difficulty of tracing the roots. When the roots are followed 

 into the encephalon, they enter masses of gray substance, which are looked 

 upon as ganglia of origin. 

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