SECT. Il8.] NERVOUS SYSTEM. 243 



lime, with phosphate of lime, magnesia, and an organic substance, which, 

 after the extraction of the salts, completely retains the form of the concretion, 

 and appears as a concentrically stratified, pale mass. It is quite certain, 

 that this brain-sand when it appears as elongated, branched or reticulated 

 masses, is simply developed in the bundles of connective tissue, as not 

 unfrequently occurs in the pineal gland and in the membranes of the brain ; 

 in other cases it appears to be an independent incrustation of fibrine- 

 coagula. On the other hand, cells impregnated with calcareous matter, do 

 not occur. Lastly, we have still to notice the glands of Pacchioni and the 

 ossifications of the cerebral membranes. The former, which are principally 

 situated at both sides of the base of the greater J'alx, and at the border of 

 the great longitudinal fissure of the brain, upon the r f!ocatli, in the choroid- 

 plexus, etc., consist chiefly of a dense fibrous substance, resembling immature 

 connective tissue, and also contain undeveloped elastic tissue, brain-sand 

 and corpuscula amylacea. The ossifications, which are true bone-plates, are 

 met with partly upon the inner surface of the dura muter of the brain, partly 

 upon the arachnoid, particularly of the cuuda equina. 



Peripheral Nervous System. 



§ 118. Nerves of the Spinal Cord. — The 31 pairs of nerves 

 coming from the cord, arise, with few exceptions, by means of 

 anterior and posterior roots. These, receiving a delicate invest- 

 ment from the pia mater, pass, converging towards each other, 

 through the sub-arachnoid space, and then penetrate, independently 

 of each other, the arachnoidea and dura mater, which latter 

 furnishes them with a firmer covering. In their further course, a 

 ganglion is formed, on the posterior roots by means of the deposi- 

 tion around and between their nerve-fibres, of ganglionic cells, 

 which, to all appearance, serve as origin to special nerve-tubes, the 

 ganglionic fibres of the sjrinal nerves. These generally arise singly 

 from cells, and have nothing in common with the fibres of the 

 posterior roots which only pass through the ganglion, further than 

 that, in their invariably peripheral course, they become applied, to 

 them, and mingle with them. The motor root never contains 

 ganglionic cells, but goes past the ganglion, after lying more or 

 less close to it. Beyond the ganglion, the two roots unite in such 

 a manner that their fibres become intimately mixed and form a 

 common nervous trunk, which contains sensitive and motor 

 elements in all its parts. The nervous trunk so formed, usually 

 becomes connected with the neighbouring upper and lower nerves, 

 to form the well-known nervous plexuses, and, finally, gives off 

 its terminal branches to the muscles, skin, vessels of the trunk 

 and extremities, to the articular capsules, the tendons and the 

 bones. As in the roots, so in the branches of the common trunk, 



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