Prof. Allex Thomson on the Spinal Cord and Nerves. 351 



greater size of the posterior than of the anterior roots of the spinal 

 nerves. He farther supposed the cells to establish communication 

 between groups of the spinal nerves, so as to co-ordinate their motive 

 influences on groups of muscles ; and finally, he stated his belief that 

 when sufficient deduction was made for such fibres as might be sup- 

 posed to remain within, or to belong to the spinal cord itself, it 

 was probable that all the true sensory fibres of the posterior roots, 

 and the volunto-motory fibres of the anterior roots, ascended through 

 the spinal cord to the basal portions of the brain, there to be brought 

 into new connections with the gray matter and medullary substance of 

 that organ, in connection with the functions of volunto-motory influence 

 and sensation. At the same time, he expressed his opinion tliat the exis- 

 tence of these communications between nerve-fibres and cell-processes, as 

 matter of direct observation, had been greatly overstated by Schroder 

 Van der Kolk, and some other authors. He gave an account of the 

 measurement of the fibres of the roots of the nerves, and of the spinal 

 cord at different parts, with estimates of the number of fibres in differ- 

 ent places, which apppeared to be confirmatory of these views. 



Dr. Thomson gave an account of micrometric measurements made by 

 Dr. Keid and himself, of the size of the nerve-fibres in the spinal marrow, 

 in the anterior and posterior roots, and in the compound nerve beyond the 

 ganglion ; and stated as the result, that very little change occurs in the 

 size of the greater number of the nerve-tubes themselves in their course, 

 but that the apparent change in the size of the roots as compared with 

 the spinal marrow, and of the compound nerves as compared with the 

 roots, depends in a great measure upon the difference in the amount of 

 sheath-substance, or adventitious tissue, which comes to be interposed 

 between and around the fibres. This is especially the case with the 

 primitive nerve-tubes when they leave the spinal marrow and pass 

 through the pia mater, and at their subsequent passage through the 

 dura mater. At the first place they receive a finer sheath- matter, which 

 runs intimately between all the nerve-fibres ; but at the second, the 

 addition of a coarser sheath-substance takes place chiefly round each 

 fasciculus, and on the exterior of the whole nerve, while the fibres 

 within each fasciculus are not more separated than before. 



Dr. Thomson stated the result of a number of measiu-ements of the 

 area of section of the whole anterior and posterior roots of the spinal 

 nerves, from which it appeared that the bulk of the posterior is usually 

 from a third to twice as large as that of the anterior ; and, further, as 

 the individual nerve-tubes of the posterior roots arc on an average about 

 a fourth smaller in diameter than those of the anterior roots, the 

 number of fibres must be generally considerably more than double, 



