Bauhaus-Universität Weimar

The Cyclopaedia of Anatomy and Physiology, vol. 3: Ins-Pla
Todd, Robert Bentley
NERVOUS SYSTEM. (Comparative Anatomy.) 
raised from the inferior 
surface of the column in 
the form of a tough rib¬ 
bon. From the sides of 
the column aponeurotic 
laminæ pass off to form 
septa of attachment be¬ 
tween the muscular bun¬ 
dles; and along the me¬ 
sial plane above the co¬ 
lumn, a similar lamina 
separates the superior bun¬ 
dles of each side, and by 
splitting below and run¬ 
ning into the sides of the 
column, forms a fibrous 
canal for the spinal cord. 
Foramina exist all along 
the sides of this canal for 
the passage of the nerves. 
A similar septum is situ¬ 
ated along the inferior part 
of the column, from the 
part where the inferior 
muscular bundles unite at 
the anus, to the extremity 
of the tail. Along the 
superior edge of the apo¬ 
neurotic septum, between 
the dorsal muscular bun¬ 
dles, and stretching from 
the anterior point of the 
vertebral column to a point 
beyond the anus, and half 
embedded between the su¬ 
perior extremities of the 
muscles, is a series of 
closed cells of a flattened 
cylindrical form, adhering 
firmly to one another by 
their bases, so as to pre¬ 
sent the appearance of a 
tube flattened on the sides 
with septa at regular dis¬ 
tances. Each of these 
cells is full of a trans¬ 
parent fluid, in the centre 
of which is an irregular 
mass of semi-opaque glo¬ 
bules, apparently cells. 
This series of cylindrical 
sacs consists of the ru¬ 
diments of inter-spinous 
bones, and probably of fin 
rays, and is attached be- 
The nervous system of low to the fibrous inter- 
Amphioxus lanceolatus. muscular septa, half co- 
a,a, the spinal cord ; vered on each side by the 
b, the first pair of lateral muscles, and en- 
nerves ; c, the dorsal ; cfosed above by the tegu- 
d, the ventral branch mentary fold which con- 
of the second pa,r. dorsal fin. 
« A similar series of cells, with the same 
relations, is situated on the ventral surface of 
the body, and stretches from the spot where 
the abdominal folds terminate, to a point 
nearly opposite the termination of the dorsal 
series. % . 
“ Nervous system. — The spinal cord is 
situated on the upper surface of the chorda 
dorsalis, enclosed in the canal formed in the 
manner above described. When the whole 
length of this canal is displayed by removing 
the muscles, and then carefully opened, the 
spinal cord is seen lying in the interior, with 
nerves passing out from it on each side. It 
stretches along the whole length of the spine, 
is acuminated at both ends, and exhibits not 
the slightest trace of cerebral development. In 
its middle third, where it is most developed, it 
has the form of a ribbon, the thickness of which 
is about one-fourth or one-fifth of its breadth ; 
and along this portion, also, it presents on its 
upper surface a broad, but shallow groove. 
The other two-thirds of the cord are not so flat, 
and are not grooved above, are smaller than 
the middle third, and taper gradually ; the one 
towards the anterior, the other towards the 
posterior extremity of the vertebral column. 
A streak of black pigment runs along the 
middle of the upper surface of the cord. It 
is situated in the groove already described, 
and is in greater abundance anteriorly and pos¬ 
teriorly, where the nerves pass off at shorter 
intervals, than at the middle or broadest part 
of the organ. From fifty-five to sixty nerves 
pass off from each side of the cord; but, as 
the anterior and posterior vertebræ are very 
minute, and run into one another, and as the 
spinal cord itself almost disappears at the two 
extremities, it is impossible to ascertain the 
exact number, either of vertebræ or of spinal 
nerves. These nerves are not connected to the 
spinal marrow by double roots, but are inserted 
at once into its edges in the form of simple 
“ The nerves pass out of the intervertebral 
foramina of the membranous spinal canal, 
divide into two sets of branches, one of which 
run up between the dorsal muscular bundles 
(dorsal branches) ; the other (ventral branches) 
run obliquely downwards and backwards on 
the surface of the fibrous sheath of the vertebral 
column; attach themselves to the antero-pos- 
terior aspect of each of the inferior muscular 
bundles, and may be distinctly traced beyond 
the extremity of each bundle. When an entire 
animal is examined by transmitted light, and a 
sufficient magnifying power, the anterior extre¬ 
mity of the spinal cord is observed, as before 
mentioned, to terminate in a minute filament 
above the anterior extremity of the vertebral 
column. The first pair of nerves is excessively 
minute, and passes into the membranous parts 
at the anterior superior angle of the mouth. 
The second pair is considerably larger, and, 
like the first pair, passes out of the canal in 
front of the anterior muscular bundle. The 
second pair immediately sends a considerable 
branch (corresponding to the dorsal branches 
of the other nerves) upwards and backwards, 
along the anterior edge of the first dorsal mus¬ 
cular bundle. This branch joins the dorsal 
branch of the third pair, and, passing on, joins 
a considerable number of these in succession, 
and at last becomes too minute to be traced 
farther. After sending off this dorsal branch, 
the second pair passes downwards and back-


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