244 Royal Society. 



blance to the sympathetic fibres or caudate processes. These fibres 

 are found through the rest of the gray substance, but in smaller 

 numbers ; immediately below the gelatinous substance, however, 

 they form a remarkably dense band. 



Changes in the form of the gray substance are observed on ex- 

 amining the cord from below upwards. The posterior mass is gra- 

 dually divided into two, and the gelatinous substance is interrupted 

 in the middle line. Two vesicular masses gradually appear at the 

 sides of the spinal canal, traversed by fibres from the posterior 

 nerves, and from the transverse commissure. The anterior horns 

 also undergo certain changes, become club-shaped in the lumbar 

 region, and contain large groups of caudate vesicles. In the middle 

 of the dorsal region the posterior horns have again coalesced, and 

 tlie gelatinous substance again crosses the middle line. The ante- 

 rior horns are straight aud narrow, and their vesicles are scattered 

 through them irregularly. In the cervical enlargement the posterior 

 gray substance has become again divided to its greatest extent into 

 two horns, and the gelatinous substance is again interrupted in the 

 middle. The two vesicular masses from the sides of the spinal canal 

 are here, as in the lumbar enlargement, included in the posterior 

 horns. Here also, and in the lumbar enlargement, the anterior horns 

 are nearly similar in appearance. 



The nucleus of vesicular substance, in which the spinal accessory 

 nerve may be seen to arise, has been traced by the author as low 

 down in the cord as the upper part of the lumbar enlargement. 



The grayish structure immediately surrounding the spinal canal, 

 consists chiefly of fibro-cellular tissue, and is not to be regarded as 

 a commissure, as maintained by Stilling and Foville. The spinal 

 canal, as stated by the former observer, extends through the whole 

 length of the cord. 



The vesicles of the cord are found chiefly in the anterior horns, 

 as usually stated, but occur also in the dark masses situated in the 

 dorsal region at the sides of the spinal canal ; and also more spa- 

 ringly in the posterior horns as far as the gelatinous substance. The 

 author has never been able to make out satisfactorily in mammalia, 

 any connection between the nerve-vesicles and the tubular nerves, 

 nor between the latter and the caudate processes. 



The blood-vessels of the cord enter through the anterior and pos- 

 terior median fissures, through the smaller fissures in the white 

 columns, and at the roots of the nei-ves. They form a beautiful net- 

 work of loops along the whole periphery of the gray substance. 



Of the WhiteColumns of the Cord. — The anterior columns have 

 no proper transverse commissural fibres, but are crossed horizontally 

 but chiefly obliquely, by tubular nerve-fibres and blood-vessels, 

 which proceed from the gray substance on each side and decussate 

 in front of the spinal canal : nor are the posterior white columns 

 connected by any commissural fibres, the posterior fissure reaching 

 down to the gray substance. 



Origin of the Spinal Nerves. — The posterior roots are attached 

 exclusively to the posterior columns. Their fibrils generally are 



