EXAMINATION OF SOME SPINAL NERVES 
95 
SECTION III.— THE SEGMENTAL ARCHITECTURE OF THE LIMB 
Owing to Goethe and Oken, the conception became established that the plan oi archi- 
tecture of vertebrate animals is that of a fore-and-aft series of structural units or segments funda- 
mentally similar one to another. Successive morphologists have, since their time, made use of 
various tissues as indices in the study of the disposition of the individual component metamers. 
The recognition of the segmental arrangement had been originally due largely to the obvious 
character of the series of vertebras and ribs. It w^as, consequently, the osseous parts that were 
chiefly appealed to in the earlier attempts at extending the segmental theory of composition of 
the body to the limb (Carus, Owen), and in endeavouring to recognize the individual segments 
in the lim.b. Osseous tissue, I would maintain, is really poorly suited to such a purpose. If the 
presence of five digits at the free end of the limb could be taken as indication that tlie number ot 
segments in the limb is five, then might the osseous tissue be useful for the study. The criteria 
used in the observations in this paper are, I think, demonstrably preferable to those taken from tlie 
bony system, and they show that such a character as the number of digits is a useless one in the 
examination of the segmental theory. The wide employment of osseous tissue for the purpose 
may, perhaps, have been in part due to the abundance of palasontological data it would contribute, 
and it is for this reason the more to be regretted that — as I believe — the osseous system, in many 
parts, is quite unfitted to throw light on the existence or position of metamers. 
GooDSiR early (1857) insisted that the nervous elements of the limb appear to indicate 
more clearly than the other of its constituents, the morphological construction of the part. None 
the less, the study of the segmentation continued to be pursued chiefly upon bones. Later, the 
musculature came in for a share of attention, and now the muscles of the limb seem to obtain a 
preponderant and, perhaps, undue amount of consideration in the matter. A difficulty in making 
use of the nerve-fibres lies, no doubt, in the existence of the complex limb-plexuses entangling 
the segmental nerves at their very outset, before they are even fully launched upon their 
distribution. This diflficulty can partly be removed by careful dissection, although, as pointed out 
by W. Krause,* dissection alone cannot adequately unravel the nerve-courses even in peripheral 
nerves. By using, however, the physiological laws of isolated conduction in nerve-fibres 
(Muller), and of degeneration of nerve-cell processes (Waller), the difficulties of the nerve- 
plexuses and nerve inosculations can be completely set aside. This being done, the nerve-elements 
in the limb become the best guides available for the tracing of segmental architecture. Tliat was 
the view of Goodsir, and I share it for the following reasons. The nerve-cells are among tiie 
earliest cells in the body to cease the exercise of reproductive activity, and especially is this true 
of the cells of the spinal ganglia and of the primary motor neurons of the spinal cord. Neither 
are these cells in any sense vagrant. Remaining from a very early period securely anchored in a 
spot of known metameric orientation, their cell-processes, i.e., nerve-fibres, radiate out into the rest 
of the metamer, whose various elements they have to bring into relationship with themselves as 
* 'Beit. z. Neurol, d. oberen Extremitiit,' Leipzig, 1865 ; see Plate. 
