NERVOUS CENTRES. (HUMAN ANATOMY. THE SPINAL CORD.) 



657 



tomoses of the two anterior spinal branches of 

 the vertebral. In one case there was no artery 

 on the left side, but the right was twice as large 

 as usual. The vessel is of considerable size 

 until it has passed below the cervical enlarge- 

 ment of the cord, from which point down 

 nearly to the lumbar enlargement it becomes 

 exceedingly delicate; a little above the last- 

 named enlargement it suddenly increases in 

 size, again gradually diminishes as it ap- 

 proaches the lower end of the spinal cord, and 

 becoming capillary is prolonged down to the 

 sacrum, together with the fibrous string in 

 which the spinal cord terminates." 



" During its course this artery receives la- 

 teral branches from the ascending cervical and 

 the vertebral in the neck, and from the spinal 

 branches of the intercostal and lumbar arteries 

 in the back and loins. Those branches pene- 

 trate the fibrous canal formed by the dura 

 mater around each of the spinal nerves ; be- 

 come applied to the nervous ganglia to which 

 they supply branches, yet intermixed with and 

 follow the course of the corresponding nerves ; 

 send small twigs backwards to the posterior 

 spinal arteries, and terminate in the anterior 

 spinal trunk at variable angles, similar to those 

 at which the nerves are attached to the cord."* 



The posterior spinal arteries arise from the 

 vertebral or from the inferior cerebellar artery: 

 they incline backwards to the posterior surface 

 of the spinal marrow, along which they descend 

 in a tortuous manner, anatomosing freely with 

 each other and with the small arteries which 

 accompany the nerves in the intercostal fora- 

 mina. A network of vessels surrounds each 

 posterior root of a spinal nerve, derived from 

 ramifications of those arteries. We can trace 

 the posterior spinal arteries as low down as the 

 lumbar region, distinct throughout their entire 

 course. 



Feiras. The blood is returned from the 

 spinal cord by a venous plexus which emerges 

 from the pia mater and is spread over its 

 whole surface : opposite the roots of each nerve 

 a small vein is formed, which passes outwards 

 with the nerve in the same sheath, and empties 

 itself into the large vein which is situate in the 

 intervertebral foramen. Veins accompany the 

 anterior and posterior spinal arteries in the 

 upper part of their course. Branches from 

 this plexus frequently pass to the dura mater 

 involved in a fold of arachnoid, and thus com- 

 municate with the general plexus which sur- 

 rounds the sheath. 



We observe that the arteries of the spinal 

 cord are reduced to a very minute size before 

 they penetrate the substance of that organ. 

 The largest vessels are therefore found on its 

 surface or in its fissures. And it may be 

 further remarked that when vessels of a size to 

 be readily detected by the naked eye penetrate 

 the substance, numerous foramina, produced by 

 the separation of the nervous fibres, become 

 distinctly visible. This is very obvious in the 

 white commissure. 



The purpose of such a minute subdivision of 



* Cruvcilhier, Anat. Descr. 

 VOL. in. 



bloodvessels, prior to their entrance into the 

 substance of the cord, must evidently be to 

 guard the nervous substance against the impulse 

 of several columns of blood of large size. A 

 similar provision, made in a more conspicuous 

 manner, is manifest in the brain, and will be 

 noticed by-and-bye. 



Of the spinal nerves. There is a pair of 

 spinal nerves for each intercostal foramen, and 

 for that between the atlas and occiput. We 

 can thus enumerate in all thirty-one pair of 

 nerves having their origin from the spinal cord, 

 and this number is exclusive of the spinal ac- 

 cessory nerve which is connected with the 

 upper part of the cervical region. 



The spinal nerves have I he following very 

 constant characters. Each has its origin by 

 two roots, of which the anterior is distinct- 

 ly inferior in size to the posterior. The 

 ligamentum denticulatum is placed between 

 these roots. Each root passes out through a 

 distinct opening in the dura mater. Imme- 

 diately after its emergence a ganglion is 

 formed on each posterior root, and the ante- 

 rior root lies embedded in the anterior surface 

 of the ganglion and involved in the same 

 sheath (Jig 378), but without mingling its 

 fibres with those of the ganglion. Beyond it, 



Fig. 378. 



A 



Origin of a spinal nerve. 

 (After Bell.) 



A, A, anterior root. - 

 P, posterior ditto. 

 G, ganglion on the posterior root. 

 C, compound nerve resulting from the commin- 

 gling of the fibres of both roots. 



the nervous fibres of both roots intermingle, 

 and a compound spinal nerve results. The 

 trunk thus formed passes immediately through 

 the intervertebral tube and divides into an an- 

 terior and posterior branch, which are distri- 

 buted to the museles and integument of the 



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