Bauhaus-Universität Weimar

Titel:
The Cyclopaedia of Anatomy and Physiology, vol. 3: Ins-Pla
Person:
Todd, Robert Bentley
PURL:
https://digitalesammlungen.uni-weimar.de/viewer/image/lit29464/625/
617 
NERVOUS SYSTEM. (Comparative Anatomy.) 
wards on each side above the hyoid apparatus, 
and joins all the ventral branches of the other 
spinal nerves in succession, as its dorsal branch 
did along the back. This ventral branch of 
the second pair is very conspicuous, and may 
be easily traced along the line formed by the 
inferior extremities of the ventral divisions of 
the muscular bundles, the ventral branches of 
the other nerves joining it at acute angles 
between each bundle. It may be traced be¬ 
yond the anus, but is lost sight of near the 
extremity of the tail. Twigs undoubtedly pass 
from the spinal and lateral nerves towards the 
abdominal surface of the body, but, on account 
of their minuteness, and the difficulty of de¬ 
tecting them in detached portions of the 
abdominal membrane, they could not be satis¬ 
factorily seen. 
“ When a portion of the spinal cord is 
examined under a sufficient magnifying power, 
it is seen to be composed entirely of nucleated 
cells, very loosely attached to one another, but 
enclosed in an excessively delicate covering of 
pia mater. The cells are not arranged in any 
definite direction, except in the middle third 
of the cord, where they assume a longitudinal 
linear direction, but without altering their 
primitive spherical form. The black pigment, 
formerly mentioned as existing more particu¬ 
larly on the upper surface and groove, is 
observed to be more abundant opposite the 
origin of the nerves; and, as it is regularly 
arranged in this manner in dark masses along 
the anterior and posterior thirds of the cord, 
the organ in these places, on superficial inspec¬ 
tion, resembles much the abdominal ganglionic 
cord of an annulose animal. Along the middle 
third the pigment is not so regular, but appears 
in spots at short intervals. When any portion 
of the cord, however, is slightly compressed, 
and microscopically examined, it becomes evi¬ 
dent that there is, along the groove and mesial 
line of its upper surface, a band, consisting of 
cells of a larger size than those composing the 
rest of the organ. Some of these cells only 
are filled with black pigment, but all of them 
contain a fluid of a brown tint, which renders 
the tract of large cells distinctly visible. When 
the compression is increased the cells burst ; 
and the fluid which flows from the central 
tract is seen to contain jet-black granules, 
which may be detected as they escape from the 
cells. 
“ The nerves consist of primitive fibres, of 
a cylindrical shape, with faint longitudinal 
striae. The primitive fibres of a trunk pass off 
into a branch, in the usual way, without 
dividing; and, where the trunks join the spinal 
cord, the primitive fibres are seen to approach 
close to it, but without passing into it. The 
greater part of the slightly protuberant origin 
consisting of the nucleated cells of the cord, 
with a few pigment cells interspersed, the 
exact mode of termination of the central ex¬ 
tremities of the primitive nervous fibres could 
not be detected/' 
We hope we may be excused for quoting the 
following additional remarks. 
“ One of the most remarkable peculiarities 
in the Lancelet is the absence of the brain. 
Retzius, indeed, describes the spinal marrow 
as terminating considerably behind the anterior 
extremity of the chorda dorsalis, in a brain 
which exhibits scarcely any dilatation; but 
careful examination of the dissection of my 
own specimen, which I have also submitted to 
the inspection of Dr. John Reid, and of other 
competent judges, has convinced me that the 
spinal cord, which may be traced with the 
greatest ease to within 1-16th of an inch of 
the extremity of the chorda dorsalis, does not 
dilate into a brain at all. It may be urged that 
we ought to consider the anterior half of the 
middle third of the spinal marrow, where it is 
most developed, to be the brain, and all that 
portion of the chorda dorsalis which is in con¬ 
nection with the branchial cavity, as the cra¬ 
nium. That this does not express the true 
relation of the parts, is evident from the fact, 
that this portion of the cord, to its very extre¬ 
mity, gives off nerves, which are too numerous 
to be considered as cerebral, but more espe¬ 
cially from the mode of distribution of the 
first and second pairs, which, in my opinion, 
proves the anterior pointed extremity to be the 
representative of the brain of the more highly 
developed vertebrata. A brain of such sim¬ 
plicity necessarily precludes, on anatomical 
grounds alone, the existence of organs of vision 
and of hearing. These special organs, deve¬ 
loped in the vertebrata at least, in a direct 
relation with the cephalic integuments and the 
brain, could not exist, even in the form of 
appreciable germs, in the Lancelet. The black 
spot which Retzius took for the rudiment of an 
eye may probably have been, what also deceived 
me at first, a portion of the black mud which 
floats about in the branchial cavity, and which 
adheres obstinately to the parts in the neigh¬ 
bourhood of the oral filaments. The first pair 
of nerves, although very minute, in accordance 
with the slight development of the parts about 
the snout, and the want of special organs of 
sense, might, from their position and relations, 
be considered as corresponding to the trifacial 
in the higher vertebrata. The second pair 
appears to be the vagus, not only from its 
distribution as a longitudinal filament on each 
side of the body, as in other fishes, but also 
from its relations to the hyoid apparatus and 
branchial cavity, to which division of organs 
the eighth pair of fishes is specially devoted. 
The distribution of a branch of this nerve, 
however, along the base of the dorsal fin, and 
the course of the posterior part of the main 
branch, would appear to shew that this nerve, 
which I have provisionally denominated the 
vagus, is, in fact, the trifacial, which, in the 
higher fishes, is not only distributed to all the 
fins, but holds exactly the same relations to the 
dorsal and anal fins, and to the spinal nerves, 
as the nerve now under consideration in the 
Lancelet. 
“ The peculiarities in the structure of the 
spinal cord are not less remarkable than those 
of its configuration. It is difficult to under¬ 
stand, according to the received opinions on 
the subject, how a spinal cord destitute of
        

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