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CXXXIX. The nervous filaments unite, or are separated from one 

 another, but do not run into each other. The divisions of the nerves are 

 different from those of the arteries, and their mode of junction does not 

 admit of being compared to that of the veins. It is, in the first instance, 

 a mere separation; in the second, an approximation of filaments which had 

 been separated, and which, though united in common sheaths, have, ne- 

 nertheless, each a proper covering, are merely in juxta-position and per- 

 fectly distinct. If that were not the case, one could not say, that each 

 fibre has one extremity in the brain, and the other in some one point of 

 the body; nor could one conceive how the impressions which several 

 sentient extremities receive at once, reach the brain without running 

 into each other: nor, in what manner the principle of motion could be 

 directed towards a single muscle receiving its nerves from the same trunk 

 as the other muscles of the limb. 



In general, the nerves divide from each other and unite at an angle 

 more or less acute, and equally favourable to the circulation of a fluid, 

 from the circumference to the centre, and from the centre to the circum- 

 ference. 



The structure of the nerves is different according to their situation. 

 Thus, the medullary fibres of the optic nerve are not furnished with mem- 

 branous coverings, the pia mater alone furnishing a sheath to the cord 

 formed by the union of these filaments; the dura mater adds a second 

 coat to it, on its leaving the skull. This coat, belonging, likewise, to the 

 whole nerve, is not continued over it, after it has entered the eye-ball, 

 and is lost in the sclerotica. A minute artery passes through the centre 

 of the optic nerve, and then dividing, forms a rete mirabile which sup- 

 ports, the medullary pulp of the retina. The nerves which pass along 

 osseous canals, as the Vidian nerve of the fifth pair, are not provided 

 with a cellular covering, and their consistence is always greater than that 

 of the nerves which are surrounded by soft parts. 



CXL. On reaching the brain, the medulla oblongata, or the spinal 

 marrow, every nervous filament, as was already mentioned, parts with its 

 membranous covering, which is lost in the pia mater, or immediate co- 

 vering of these central parts of the brain. The medullary or white part 

 of the brain is continued into their substance, which may be considered 

 as principally formed by the assemblage of these nervous extremities 

 which it is difficult to distinguish in its tissue, from its want of consist- 

 ence. It has long been known, that the origin of the nerves is not the 

 spot at which they are detached from the brain, that they sink into the 

 substance of this viscus, in which their fibres cross each other so that 

 those on the right pass to the left, and vice versa. Scemmering thought, 

 that the roots of the nerves, especially of the nerves of the organs of 

 sense, reached to the prominences in the parietes of the ventricles of the 

 brain, and that their furthest extremity was moistened by the serosity 

 which keeps these inward surfaces in contact. It has likewise long been 

 thought, that the cerebral extremities of the nerves all joined in a fixed 

 point of the brain, and that to this central point all sensations were car- 

 ried, and that from it all the determinations producing voluntary motion 

 arose. But the inquiries of Gall on the structure of the brain and ner- 

 vous system, have completely overset these various hypotheses. 



The spinal marrow and the nerves, in the different animals furnished 

 with them, are larger in proportion to the brain, according as the animal 

 is more distant from man in the scale of animation. In carnivorous 



