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Part V.
ANATOMY. PART OF
THE
Sect. I. Of the Nerves in general. THE medullary fubftance of the brain is employed in forming the white fibrous cords, which are called nerves. Within the fkull we fee the nerves to be the medullary fubftance continued; and the fpinal marrow is all employed in forming nerves. The nerves are compofed of a great many threads, lying parallel to each other, or nearly fo, at their exit from the medulla. This fibrous texture is evident at the origin of moft of the nerves within the fkuil; and in the cauda equina of the fpinal marrow we can divide them into fuch fmall threads, that a very good eye can fcarce perceive them; but thefe threads, when looked at with a microfcope, appear each to be comppfed of a great number of fmaller threads. How fmall one of thefe fibrils of the nerves is, we know not; but when we confider that every, even the moft minute part of the body is fenfible, and that this muft depend on the nerves, (which all conjoined would not make a cord of an inch diameter), being divided into branches or filaments to be difperfed through all thefe minute parts, we muft be convinced, that the nervous fibrils tre very fmall. The medullary fubftance, of which the nervous fibrils are compofed, is very tender, and would not be able to refill fuch forces as the nerves are expofed to within the bones, nor even the common force of the circulating fluids, were not the pia mater and tunica arachnoides continued upon them; the former giving them firmnefs and ftrength, and the latter furnilhing a cellular coat to conned the threads of the nerves, to let them lie foft and moift, and to fupport the vefiels which go with them. It is this cellular fubftance that is diftended when air is forced through a blow-pipe thruft into a nerve, and that makes a nerve appear all fpongy, a'fter being diftended with air till it dries ; the proper nervous fibrils ihriveiling fo in drying, that they fcarce can be obferved. Thefe coats would not make the nerves ftrong enough to bear the ftretching and preflure they are expofed to in their courfe to the different parts of the body ; and therefore, where the nerves go out at the holes in the cranium and fpine,.the dura mater is»generaliy wrapt clofely round them, to colled their difgregated fibres into tight firm cords ; and that the tenfion which they may happen to be expofed. to may not injure them before.they have got
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V.
NERVES. this additional coat, it is firmly fixed to the fides of the holes in the bones through which they pafs. The nervous cords, thus compofed of nervous fibrils, cellular coat, pia and dura mater, have fuch numerous blood-veflels, that, after their arteries only are injeded, the whole cord is tinged of the colour of the injeded liquor. A nervous cord has very little elafticity, compared with feveral other parts of the body. When cut out of the body, it does not become obfervably fhorter, while the blood-veflels contrad three eighths of their length. Nerves are generally lodged in a cellular or fatty fubftance, and have their courfe in the interftices of mufcles, where they are guarded from preflure; but in feveralparts they are fo placed, as if it was intended that they. Ihould there fuffer the vibrating force of arteries, or the preflure of the contrading fibres of mufcles. The larger ccrds of the nerves divide into branches as they go off to the different parts; the branches being fmaller than the trunk from which they come, and making generally an acute angle where they feparate. In feveral places, different nerves unite into one cord, which is commonly larger than any of the nerves which form it. Several nerves-, particularly tliofe which are diftributed to the bowels, after fuch union, fuddenly form a hard knot confiderably larger than all the nerves of which it is made. Thefe knots were formerly called corpora olivaria, and are now generally namtAganglions. The ganglions have thicker coats, more numerous and larger blood-veflels than the nerves ; fo that they appear more red and mufcular. Commonly nurnerous fmall nerves, which conjundly are not equal to the fize of the ganglion, are font out from it, hut with a ftrudure no way different from that of other nerves. The nerves fent to the organs of the fenles, lofe there their firm coats-, and terminate in a pulpy fubftance. The optic nerves are expanded into-the foft tender, webs, the retinae. The auditory nerve has fcarce the confiftence of mucus in the veftibulum, cochlea,, and femicircular canals-of each ear. The papillae of the nofe, tongue, and flein, are very foft. The nerves of'mufcles-can likewife be traced' till they feem to lofe their -coats by becoming very fdft ; from which, and what we obferved of the fenfatory nerves, there is reafon to conclude, that the mufcular nerves are alfo pulpy at their terminations, which we cannot indeed. profecute by diffe&ion. Sect..
