138 THOMPSON YATES LABORATORIES REPORT 
laid down to be a traction of spinal cord included between two imaginary frontal sections, placed 
one on each side of a nerve-pair, and each half-way between the next pair below and the next 
above. Neither morphologically nor physiologically does this cylindroid mass of the cord merit 
the term ' spinal segment.' To take such a fraction as representing either a structural or a 
functional unit or spinal link, from a series of which the cord is built up by concatenation, is to 
ignore one of the most important characters of construction of the myelon. Overlapping and 
inlapping of segments exist in the segmental arrangement of the cord itself just as in the segmental 
arrangement of the neurons in the body-metameres outside. The 'block' delimitation of the 
spinal segment is no longer defensible or useful. It omits structures which are essential 
components of every complete spinal segment ; it includes much that is extraneous. It is 
applicable, probably, to one set only of the spinal neurons, namely, to the motor neurons, for, as 
I* and A. S. Grunbaum* have shown, the spinal motor root is of strictly local origin. 
The extent of intra-spinal attachment is very different in the two cases of the dorsal 
(afferent) and of the ventral (efferent) root respectively. The former overlaps a series of the latter. 
If, for mere convenience of statement, the surface attachment only of the roots is considered in 
the definition ' segment,' there remains, apart from the obvious artificiality of the postulate, still a 
difficulty witli the dorsal (afferent) root. This difficulty lies in the frequency with which the 
filaments of dorsal roots trespass upon each other's territory, a filament from one root-ganglion not 
uncommonly joining a filament from the next ganglion to plunge together with it into the postero- 
lateral fissure of the cord. Not very rarely a filament on issuing from the ganglion pursues an 
oblique course (generally in my experience downward) across a part, or even the whole of the 
filaments from the next ganglion, to enter the cord among or even subjacent to them. 
Example. — I recollect noting in the human brachial enlargement a filament from the Vllth 
cervical ganglion plunging into the cord half-way down the series of filaments from the Vlllth ganglion. 
I believe such variations are not found in the filaments of anterior roots. I have myself, with considerable 
opportunity for search, never yet found any. 
The surface attachment of the dorsal (afferent) root is, therefore, just as its deep attachment, 
a less reliable segmental guide than that of the ventral (efferent) root. But the collection of cells 
composing the spinal ganglion is a segmental collection, just as the collection of the intra-spinal 
neurons of the efferent root is a segmental group ; indeed, the two are segmentally equivalent. 
To include in the 'spinal segment' the cells of one root, and to omit those of the other, is 
obviously artificial and inconsequent. The spinal segment is, therefore, in this paper understood 
to include the neurons of the spinal ganglion, as well as those of the corresponding ventral 
(efferent) root, and along with these all other intra-spinal neurons whose cell-bodies lie between 
the same frontal levels as the neurons of the efferent root in question. I omit reference to the 
ganglia of the sympathetic, this paper not being immediately concerned with them. 
As to 'short paths,' KoLLiKERt defines them as paths which do not extend the whole 
length of the spinal cord and do not extend into the brain. I would propose to limit the term 
* 'The Journal of Physiology,' vol. 13, p. 707, 1892 ; vol. 14, p. 300, 1893 ; vol. 16, p. 368, 1894. 
t ' Handbuch der Gewebelehre iles Menschen,' 6th edit., vol. 2, ist half. Leipzig, 1893. 
