650 



NERVOUS SYSTEM. (NERVOUS CENTRES. THE SPINAL CORD.) 



anatomy, to determine whether there are any 

 nerve-tubes which terminate in the grey matter of 

 the ganglion, or originate in it, which in short 

 are not continued through the ganglion. At 

 present we are unable to state further than that 

 the tubes appear to have an intimate connection 

 with the nerve-vesicles wherever the latter may 

 be found, and that they often appear to be con- 

 tinuous with the sheaths of the nerve-vesicles. 



Fig. 375. 



Second abdominal ganglion of a greenfinch, slightly 

 compressed under the compressor. The course of the 

 nerve-tubes only is represented, 



a, fibres passing in ; b, emerging fibres ; c, sur- 

 rounding fibres. The meshes for the reception of 

 ganglion globules are shown. 



Fig. 374. 



A small piece of the otic ganglion of the sheep, slightly 

 compressed, showing the interlacement of the inter- 

 nal fibres and, the grey matter. 



(After Valentin.) 



tin, travel round the margin of the ganglion, 

 and to these he gives the name of umspinnende 

 Fasern, surrounding fibres, and some fibres 

 pass from them to the more central ones, or 

 from the latter to the former. Nerve-vesicles 

 exist at the circumference of the ganglion as 

 well as in its interior, and to them is due the 

 peculiar grey colour of that body. 



The best mode of examining these points is 

 to select the smallest ganglia of very small 

 animals, birds, mice, &c. ; these, when sub- 

 jected to compression, become very transpa- 

 rent, and display much of their intrinsic ar- 

 rangement. Or thin slices of large ganglia may 

 be placed under the microscope, and when torn 

 up by needles the disposition of the nerve- 

 vesicles and the caudate processes, when pre- 

 sent, are rendered visible. And none is more 

 suitable for this purpose than the Casserian 

 ganglion of the fifth nerve, which by the ab- 

 sence of a dense sheath and its greater loose- 

 ness of texture is more easily examined. 



It is a highly important problem, in minute 



Nerve vesicles from the Gasserian ganglion covered by 

 t/teir sheaths of nucleated particles, to shew the inti- 

 mate relation of the nerve-tubes to them. 



t, t f nerve-tubes ; v, v, vesicles. 



There does not appear to be any material 

 difference of structure between the ganglions of 

 the sympathetic and those of the cerebro-spinal 

 system, excepting, as Henle states, the exist- 

 ence of a greater number of gelatinous fibres 

 in the former. 



Of the cerebro-spinal centre. The nervous 

 mass which occupies the cavities of the cra- 

 nium and spine doubtless constitutes one great 

 centre, as there is a perfect continuity through- 

 out all its parts. But the differences of external 

 form and characters in some regions of it, and 

 the obvious diversity of endowment of the nerves 

 connected with certain portions, denote and 

 justify an anatomical as well as a physiological 

 subdivision of it into segments, each of which 

 is a centre of nervous action independent of the 

 rest, yet so connected with them that the func- 

 tions of all are made to harmonize in the most 

 perfect manner. 



The subdivision which the external anatomy 

 indicates, although not perfectly coincident with 

 that which the differences of function would 

 suggest, has been so long sanctioned by usage 

 and is so convenient for description, that no 

 advantage would be gained by adopting any 

 other. Our description of the cerebro-spinal 

 centre, or axis as it has also been called, will 

 be given under the following heads : 1 . the 

 spinal cord ; 2. the encephalon, including , the 

 medulla oblongata; b, the mesocephale; c, the 

 cerebellum ; d, the cerebrum. 



1. OF THE SPINAL CORD. Syn. Spinal mar- 

 row, Medulla spinalis; Fr. La motile epiniire ; 

 Germ. Das Riicken/nark. The following are 

 the anatomical limits which may be assigned to 

 the spinal cord. It occupies a large portion of 

 the spinal canal, terminating inferiorly at a 

 point which, in different subjects, ranges be- 

 tween the last dorsal and the second lumbar 

 vertebra. Below this point the sheath formed 

 by the dura mater contains that leash of 

 nerves which is called the cauda equina, in 



