NERVOUS CENTRES. (HUMAN ANATOMY. THE ENCEPIIALON.) 



673 



of the parts. The whole of that portion of 

 the cerebral hemisphere which is situate behind 

 the Sylvian fissure should be called the pos- 

 terior lobe. 



The hollow space between the middle lobes 

 of the brain corresponds to the principal ante- 

 rior reservoir of subarachnoid fluid. It is 

 situate immediately above the Sella Turcica, 

 and, indeed, the brain is, as it were, tied to the 

 pituitary body, which is firmly lodged in this 

 excavation ot the sphenoid bone, by a funnel- 

 shaped hollow process of nervous matter, called 

 pituitary process or tube, Cm, l,Jig. 382), which, 

 enveloped in a sheath of arachnoid membrane, 

 is inserted into it by its small extremity. This 

 space communicates with the anterior fissure 

 in the middle, and with the Sylvian fissure on 

 either side. 



Commencing at the anterior fissure and pas- 

 sing backwards, we notice the following parts, 

 to see which clearly it is necessary that the 

 adherent pia mater and the arachnoid should 

 have been previously carefully dissected away. 

 The anterior fissure is limited by the anterior 

 fold or reflection of the corpus callosum : 

 behind this we find a thin layer of a lightish 

 grey matter, which, like a triangular plate, 

 seems to stop up the third ventricle at its in- 

 ferior surface. This, indeed, which is called 

 tuber cinereum, constitutes a principal part of 

 the floor of that ventricle. The pituitary pro- 

 cess is continuous with and is probably an 

 extension of it. A probe introduced into the cut 

 extremity of this process will be found to pass 

 readily into the third ventricle. 



Immediately in front of the pituitary process, 

 the union of two white bands, which form la- 

 teral boundaries to a large portion of the tuber 

 cinereum, the optic tracts, takes place along 

 the middle line. This forms the commissure 

 of the optic nerves, from which these nerves 

 diverge. Behind the pituitary process the tuber 

 cinereum extends back to two small pisiform 

 bodies of an extremely white colour on their sur- 

 face, corpora mamillaria or nlbicuntia (m,jig. 

 382). These, we shall see by-and-bye, are con- 

 nected with one of the most important of the 

 cerebral commissures, namely, \hejbrnix. 



Behind the mamillary bodies we find a deep 

 depression into which the pia mater sinks, car- 

 rying with it very numerous bloodvessels. This 

 depression lies between two thick processes of 

 fibrous matter, which, traced from below, pass 

 upwards and outwards, expanding as they 

 advance, and upon which each hemisphere is 

 placed (to use Reil's simile) like a mushroom 

 on its stalk. These are the crura cerebri, the 

 peduncles of the cerebral hemispheres. The 

 depression above described, which separates 

 them, is the intercrural or interpeduncular 

 space. When the pia mater has been removed 

 from it, its surface appears cribriform from the 

 perforations of the numerous minute vessels 

 which penetrate it; it has been named by Vicq 

 d 'Azyr substantiu perforate media. The nervous 

 matter which forms the floor of this space has a 

 greyish hue, and connects the crura to each other, 

 like a bridge, whence the designation pans 

 Tarini. At the interpeduncular space we see 



SOL. III. 



the third pair of nerves emerging from their 

 connexion with the crura cerebri. 



The inner margin of each middle lobe of the 

 brain is separated from the corresponding crus 

 cerebri by a fissure which passes from behind 

 forwards, and terminates in the fissure of Syl- 

 vius. If this fissure be followed backwards, it 

 will be found to become continuous with a 

 transverse fissure which separates the cerebrum 

 from the cerebellum, and corresponds to the 

 posterior edge of the corpus callosum. A con- 

 tinuity is thus established between the lateral 

 and the transverse fissures, whence results one 

 great fissure of semicircular form, the concavity 

 of which is directed forwards. This is the 

 great cerebral fissure of Bichat, or the great 

 transverse or horizontal fissure (Cruveilhier.) 

 It may be described as commencing at the fissure 

 of Sylvius on one side, turning round the oppo- 

 site cerebral peduncle, and ending at the oppo- 

 site Sylvian fissure. The anterior and lateral 

 portions of this fissure have already been no- 

 ticed as the situations at which the pia mater 

 enters the brain to form the choroid plexuses of 

 the lateral ventricles. And it may be remarked 

 here, how freely the subarachnoid fluid may pass 

 along this fissure from before backwards. Pa- 

 rallel to this fissure we find the fourth pair of 

 nerves as it passes to its point of exit from 

 the cranium. 



Not the least interesting and important of the 

 objects presented at this central portion of the 

 base of the brain is that remarkable arterial 

 anastomosis, called the circle of Willis. This 

 will be more particularly described by-and-bye; 

 but it may be stated here, that the anterior 

 bifurcation of the basilar artery is immediately 

 behind the interpeduncular space, on each 

 side of which the posterior cerebral artery 

 passes for a short distance. The posterior 

 communicating artery is parallel to the inner 

 edge of the middle lobe ; the subdivision of 

 the carotid corresponds to the commencement 

 of the Sylvian fissure; and the anterior com- 

 municating artery is at right angles with the 

 longitudinal fissure immediately behind the 

 anterior reflection of the corpus callosum. 

 This anastomosis of arteries is bathed in the 

 liquid which occupies the subarachnoid space 

 in this situation. 



The tentorium cerebelli is situate on a plane 

 a little beneath that of the middle segment of the 

 base of the encephalon just described. It forms 

 a septum between the posterior lobes of the ce- 

 rebral hemispheres, which are continuous with 

 the middle segment, and the posterior segment 

 of the encephalon, which we now proceed to 

 describe. 



The posterior segment, as occupying the pos- 

 terior fossa of the cranium, is on a level con- 

 siderably below that of the middle segment. 

 The parts which are deserving of more par- 

 ticular notice here, are, proceeding from before, 

 the pons Varolii (, jig. 382), the inferior and 

 anterior surface of the mesocephale, winch is 

 situate immediately behind the interpeduncular 

 space, the crura cerebri appearing to emerge just 

 above its anterior border. From its posterior 

 edge the medulla oblongata (o) extends down- 



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