NERVOUS SYSTEM. 327 



sacrum. Around its termination come off several large nervous 

 chords, which are continued along the canal, and extend, some 

 of them, into the coccyx : altogether, they have been resembled 

 to a horse's tail, and thence denominated cauda equina. 



In the year 1808, Mr. Sewell conveyed, in a letter to Sir E. 

 Home (which was afterwards read to the Royal Society), the par- 

 ticulars of what he conceived to be a canal running from the 

 fourth ventricle through the whole length of the spinal marrow, 

 containing a limpid colourless fluid, and lined by the membrana 

 arachnoides, or a membrane very like it; but, since this, M. Bellen- 

 geri, of the College of Medicine of Turin, has proved that this 

 supposed canal is, in truth, a median furrow, dividing the spinal 

 marrow into six columns : he has likewise shewn, that the roots 

 of the spinal nerves, both anterior and posterior, are triple. More 

 recently, Sir Charles Bell has discovered, that of the three tracts 

 or columns contained in each lateral portion, the inferior column 

 is for motion, the superior for sensation, and the middle one for 

 respiration. The two former extend up into the brain, and are 

 dispersed or lost in it; but the latter stops short in the medulla 

 oblongata. 



Origin of Nerves. — The nerves given off by the spinal marrow 

 are twenty-nine pairs. They originate from the superior and in- 

 ferior columns, by superior and inferiory'oscgs, or bundles of fila- 

 ments, piercing the theca vertebralis in several places, from which 

 they derive sheaths in their egress. The cervical and dorsal nerves, 

 for the most part, arise from the marrow opposite to the foramina, 

 through which they make their exit from the spine ; but the pos- 

 terior lumbar and sacral nerves proceed obliquely backward be- 

 fore they turn out ; and some of the latter, and all the coccygeal 

 nerves, run longitudinally down the canal before they quit it. The 

 filaments composing the fasces are themselves small ; they bear 

 a proportion in size and number to the nerves of which they are 

 the roots. Threads of communication may be seen, oftenest 

 in the neck, running from one fascis to another. The supe- 

 rior fasces are separated from the inferior ones by a strong, 

 white, fibrous band, which runs along the side of the marrow, 

 and has a puckered attachment in one particular place to the 

 theca, between every pair of nerves, on either side ; inwardly, 

 it being intimately joined to the pia mater. This band, which is 

 denominated, from its several pointed connexions, the ligatnentum 

 denticulatum, prevents any such injurious extension of the nerves, 

 when the spine is forcibly flexed, as might occasion laceration of 

 their medulla. In the space between the vertebr-ae, the superior 

 set of filaments converge and form a ganglion, from the opposite 

 side of which many filaments arise, and proceed with the inferior 

 and join them, and thus form the trunk of the nerve. 



