616 
raised from the inferior 
surface of the column in 
%y = the form of a tough rib- 
Z ~— bon. From the sides of 
= the column aponeurotic 
lamine pass off to form 
septa of attachment be- 
tween the muscular bun- 
dies; and along the me- 
sial plane above the co- 
lumn, a similar lamina 
separates the superior bun- 
dies 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 
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 toa 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 
i bones, and probably of fin 
Fig. 350. rays, and if attached be- 
The nervous system of low to the fibrous inter- 
Amphiosxus lanceolatus. muscular 1 pe half co- 
a,a, the spinal cord; vered on each side by the 
b, the first pair of Jateral muscles, and en- 
Beret ical etal losed above by the tegu 
. : mentary fold which con- 
“a tales stitutes | the 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 
WI 
a 
= — 
MT 
TI 
NERVOUS SYSTEM. (Comparative Anatomy.) 
situated on the upper surface of the chorda 
dorsalis, enclosed in the canal formed in the 
manner above on Ply whole | 
length of this canal is displa emoving: 
aaa and then carefully opened, he 
spinal cord is seen lying in the interior, » 
nerves passing out from it on each side. I 
stretches along the whole length of the spi 
is acuminated at both ends, and exhibits 
the slightest trace of cerebral development. 
its middle third, where it is most develope 
has the form of a ribbon, the thickness o h 
is about one-fourth or one-fifth of its breadth; 
and along this portion, also, it poe 
ae surface © hnlal but low groove. 
e 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 thi 
middle of the upper surface of the cord. 1] 
is situated in the groove already described 
and is in greater abundance anteriorly and p 
teriorly, where the nerves off at short 
intervals, than at the middle or broadest par 
of the organ. From fifty-five to sixty ner 
pass off from each side of the cord; but, : 
the anterior and posterior vertebra are yer 
minute, and run into one another, and as tl 
spinal cord itself almost disappears at the 
extremities, it is impossible to ascertain 
exact number, either of vertebre or of s 
nerves. These nerves are not connected to 
spinal marrow by double roots, but are in 
at once into its edges in the form of 
cords. q 
“ The nerves pass out of the inte nt 
foramina of the membranous spinal can: 
divide into two sets of branches, one of wl 
run up between the dorsal muscular bund! 
(dorsal branches); the other (ventral branch 
run obliquely downwards and back 
the surface of the fibrous sheath of the 
column ; attach themselves to the antero-j 
terior aspect of each of the inferior museu 
bundles, and may be distinctly traced b 
the extremity of each bundle. When an ¢ 
animal is examined by transmitted light, a 
sufficient magnifying power, the anterior € 
mity of the spinal cord is observed, as be 
mentioned, to terminate in a minute fila 
above the anterior extremity of the vert 
column. The first pair of nerves is 
minute, and passes into the membranous: 
at the anterior superior angle of the mé 
The second pair is considerably larger, 
like the first pair, passes out of the eane 
front of the anterior muscular bundle. ~ 
second pair immediately sends a con: 
branch (corresponding to the dorsal bran 
of the other nerves) upwards and backwi 
along the anterior edge of the first dorsal m 
cular bundle. This branch joins the dor 
branch of the third pair, and, passing on, jo 
a considerable number of these in success 
and at last becomes too minute to be ff 
farther. After sending off this dorsal brant 
the second pair passes downwards anc 
’ 
