692 FUNCTIONS OP THE CEREBRO-SPINAL NERVOUS SYSTEM. 



lamina (optic radiations of Gratiolet). This mass associates itself with the posterior 

 and cxlcriidl fasciculi of the cru-ita of the crus cerebri. But the most external fascic- 

 ulus of the crusta, as just stated, passes through the decussation of the pyramids of 

 the medulla oblongata, into the posterior column of the spinal cord, and consequently 

 the posterior roots of the spinal nerves are represented in the same lohes of the hemi- 

 spheres as the radiations of the optic tract. It is remarkable that these lobes also 

 receive the fibres proceeding from the olfactory bulb and lobe. Another group of 

 fibres from the cortex pass to the corpora quadrigemina, forming the brachia of the 

 nates and testes. 



The pineal gland is a ganglion of origin of the tegmentum, and is placed over the 

 decussation of the tegmenta of the opposite sides. 



At the level of the pons is a large and important region where the brachia of the 

 cerebellum interlace with the projection system, and from this point the third link of 

 the projection system descends. The superior peduncle is composed partly of fibres 

 of the corona radiata, proceeding directly from the cortex of the cerebral hemi- 

 spheres i and partly of the fibres from the optic thalamus and corpus striaturn, all of 

 which decussate so that the superior peduncles form a horseshoe closed in front which 

 corresponds with the nates and testes. They constitute the connecting system of the 

 cerebellum. The middle peduncle of the cerebellum is best understood by remem- 

 bering that every fasciculus of the crusta terminating in the anterior division of the 

 pons is represented by two fasciculi of the opposite middle peduncle of the cerebel- 

 lum, of which one runs through the superficial, the other through the deep layer of 

 tran-verse fibres of the pons. 



The inferior peduncle of the cerebellum consists of fibres passing from the cerebel- 

 lum to the posterior columns of the spinal cord, and includes 1. The corpus resti- 

 forme; 2. The internal division of the interior crus cerebri ; and 3. The fasciculus 

 cuneatus and slender fasciculus 



The gray substance of the central cavities is a centre from which a large number 

 of nerve-roots take origin. It lines the third ventricle, surrounds the aqueduct of 

 Sylvius, expands itself in the cavity of the fourth ventricle, and incloses the central 

 canal throughout the spinal cord; from it the third, fourth, and succeeding cerebral 

 and all the spinal nerves in succession take origin, and these constitute the third link 

 of the projection system. The third nerve may in fact be taken as the type of the 

 projection system of Meynert. The first link consists of fibres extending from the 

 cerebrum to the lenticular nucleus; the second link from the lenticular nucleus to 

 the nucleus of origin of the third nerve in the gray substance of the third ventricle 

 of the opposite side, and the third link being the nerve itself. 



The cerebellum represents a loop or sling, the curves and sinuosities of which 

 establish very indirect connections between the cerebrum and the nerve-roots. The 

 auditory nerve is almost entirely lost in it, and it has extensive connections with the 

 optic nerve. The crusta of the cerebral peduncle, which descending, is continued 

 into the pyramids of the medulla oblongata, is at least three times the size of the 

 pyramids, and this diminution is due to the termination of many of the fibres of the 

 crusta in the ganglion cells of the pons, from whence fibres arise that pass into the 

 middle peduncles of the cerebellum. A motor function must therefore be attributed 

 to the cerebellum, even from this point of view alone. For no other view can be 

 taken than that the impulses conducted from the cortex to the corpus strialum and 

 the lenticular nucleus, besides their conduction into the spinal cord, at the same time 

 give rise to definite modes of activity of the cerebellum. 



The medulla oblongata consists of a prolongation of the anterior and posterior 

 tract of the crus cerebri, and of the crus cerebelli, together with much scattered gray 

 matter in its substance, and a layer on it> surface, the gray matter of the fourth ven- 

 tricle representing the gray matter of the central cavities. 



560. The weight of the entire Encephalon in the adult Male usually 

 ranges between 40 and 60 oz., the average being about 50 oz. ; and in the 

 Female from 36 to 50 oz., the average being about 45 oz. 1 The maximum 



' The average of both sexes differs, however, in the various races of mankind. 

 Dr. Davis ( Proceedings of the Koyal Society, Jan. l>:!d, 1868) found the mean of the 

 European series to be 4G.87 07. ; 'of the Asiatic series, 44. (i2 oz. ; of the American 

 series, 44. 73 ox. ; of the African series, 44.3 oz. ; and of the Australian series, 41 38 

 OK. Dr. Thurnam (Journal of Mental Science, April, I860) gives 49 oz as the 

 average weight of the European brain, whilst in distinguished men it amounts to 

 54.0 oz. 



