Page:The New International Encyclopædia 1st ed. v. 14.djvu/422

* NERVOUS SYSTEM AND BRAIN. 372 NERVOUS SYSTEM AND BRAIN. ot" the same ami ijjtpuailu sidus ut" thu curd. ^lany of tilt" tibi't'S of this tract are probalily spinal assouiatiou libres ami reenter the gray matter to terminate there. Other libres, however, pass upward to higher centres, their ex- act terminations being as yet undeter- mined. The remainder of the wliite mat- ter of the cord is constituted by the short libre systems (fundamental columns or ground bundles). It will be seen that in the main thc.-^c short libres lie adjacent to the gray matter and arc more extensive in the anterior part of the cord. They are axones of cells situated in the gray matter of the cord. These axones enter the white matter, pass up or down, or, splitting, send one branch up. the other down, and finally terminate in the gray matter. .Along their course they send collaterals into the gray matter. These fibres of the fundamental columns thus serve to connect the gray matter of different levels of the cord and may be considered of the nature of longitudinal commissural fibre sys- tems. By means of these neurones an impulse coming into the cord by the posterior nerve root may lie conveyed to many motor or sensory cells within the gray matter. In this way are accom- plished the more complex reflex actions. scribed arc lhu>c of the aiilcriur liorns con- nected with the motor spinal nerve roots. These arc large multipolar cells rich in chromatic sub- Perinai. ■Endoneu. ISl.ves. Med.slv. Fib.s7i.. POKTIOS OF A CROSS SECTION OP THE BVMAX MEDIAL NEKVE. PeriDCH., periueurium : Enrfoneu., eiuloiipuriuni; HI. res., blood- vt'Hsel containing blood eorpnsL-les; Fiit. nh., fibre sheatb; Ax. cyi., axis i-ylindor; Med. sli., medullary sheatli. stance. They are arranged in columns, two long columns extending the entire length of the cord for the innervation of the muscles of the trunk, while in the cervical and lumbar enlargements extra columns are added for the supply of the muscles of the exlreniities. The Spinal Xerves. These are the peripheral nerves connected with the spinal cord. There are 31 pairs of spinal nerves, which arc named from the levels of the cord from which they originate: 8 cervical, 12 dorsal. 5 lumbar. ,5 sacral, and 1 coccygeal. Each spinal nerve is connected with the cord by two roots, an anterior or motor root and a posterior sensory riwt.. The anterior root consists of. a.xones of the motor cells of the ante- rior liorns. These pass out in small bundles, by the ganglion of the posterior riwit, beyond which they join the fibres from the ganglion to form « ' ^ RtiAveiii 8CHEMK OF THE eTRfCTURE OF THE SPINAL CORD. a. Fa,srlriiliiH ctTebro-spinalis ventruHH; b, fartciculiis ven- trnliH proprluH : c. fayclculuH veiitrn-lateralis (iowersi; rf, fanclculiis eerebello Bpinalis: e, fasciouhiM rproln-o-Hplnalla lat^'valiH; f, fascIcuIUH lrtt«Tali8 jirnpriua; g. fimicuhis diir- 8ali>*: Hud. rior,. radix linrsalis; llatl. vent., radix vt'iitrallB; (Snng. spia.,^&ii£Un sphialf; 1. rtminiiwHuml rt-ll. (trlietiero- iiitTlL- lU-Mironert: ono Is seen eendhiK its axdininto tin' gray HubHtanco of tho other sido. tlio otlu^rH Ni'ud t hflr axmies into tin? whlto matt«'r of tlio oi>po8ite side; '1, nmtorcellH; Htdt's of flbrllM are Been arlsiiiK from their axonoH; 3. taiito- intTic iiouronew. the axonen koIuk: to the ventral and latfrnl funleuH; anions tlicwe arp cells in the nucleus dnrHjilis and sfini*' of the cells in the Hubntaiitia Kelatlnosa of Uolando. collateralH are enniiiij^ off from the iixunes; 4, a (JoIk' •""•H. or ilendraxone : 5, cells sending their axones to the dorsal funiculi ; (i. cell bodies of peripheral sensory neurfnies slt- uat*>d in the ^anKllon spinaje; their central prolonf::ations are shown enterinj? the spinal cord as dorsal root fibres whieh tufurcutjiiff send collaterals to t»'rniiriate In different parts of the substantia trrisea; 7. collaterals and t4Tniln- als n'|»res"ntin»c fibres from the fawcleull cerebro-spinnles or pyramidal tnut; ft, collaterals from the whit<' fibres In the ventral and lattTal funiculi. Xkrve C'klls of T!IK Couii. In ilcscrihint; the orij,'in of fibro systems of the cord, many of the cells of the cord have been necessarily referred to. o.R. thoHe cells the axones of which form the columns of (Jowcrs and of Fleehsip, and the fund:iMu>ntal columns. There are also found in the iT matter cells which have short axones which never leave the f;ray matter, Init terminate in the vicinity of their cells of oripin. The most important groups of cells remaining to be de- ^7 ^{ec(. cmLmin. ■ y^J .J'o/fyjHitjlossat ^5^^r^V;.^_j^ Orcipi/oFrofUali^ Stemo-Masloid Auriadar ^Facial purl-:"'' CLAN OK CERVICAL PLEXl'B. t!ie mixed spinal nerve. The fibres of the sensory root have been already described in connection with the posterior columns. These are the cen- tral processes of the spinal ganglion cells. The peripheral processes of these cells pass out of the outer end of the ganglion and join the motor tibres. The nerves leave the spinal canal througii the intervertebral foramina, beyond which each nerve divides into two branches, a posterior and an anterior. The posterior divisions of the spinal nerves are