_ The medullary fibres of the optic thalami (fig. 
182, ¢) and bigeminal bodies (r, s) form a thin 
stratum above a third ventricle of unusual capa- 
city, the relative size of which appeared somewhat 
Jarger than was natural from the decomposition 
of the medullary matter of the soft commis- 
sure. The principal commissure of the he- 
mispheres is the anterior one, which is sub- 
cylindrical, and measured two lines thick verti- 
cally, and one anda half horizontally. The pos- 
terior commissure is a narrow strip of medullary 
matter, which thickens the upper part of the 
valvula Vieussenii. The ‘ iter’ or canal from 
the third to the fourth ventricle is proportion- 
ally wide. The arbor vite, as displayed by 
a vertical section of the vermiform process, 
sends off four principal and some minor 
medullary branches. 
The spinal chord in the Ornithorhynchus is 
long and slender, but fills closely the spinal 
canal: it is thickest at its commencement and 
at the lower two-thirds of the cervical region ; 
itis more slender in the dorsal region, espe- 
cially near the loins; it is slightly enlarged in 
the lumbar region, and gradually terminates in 
a point in the canal of the sacral vertebra : 
the cauda equina is very feebly represented. 
“In the Echidna the form and proportions of 
_the spinal chord (fig. 185) are strikingly dif- 
ferent: it is here nearly as short and thick, 
relatively, as in the hedge-hog, and terminates 
_ ina point, at d, before it has reached the middle 
of the dorsal region. Nevertheless, in this 
short tract the two usual enlargements, giving 
origins respectively to the nerves of the pec- 
__ toral and pelvic extremities, are clearly marked ; 
__ theslightly contracted intermediate portion being 
extremely short: the cauda equina is remark- 
_ able for its length. The nerves escape, as usual, 
from the intervertebral foramina, and have a 
longer course in the spinal canal, in proportion 
as they supply parts more distant from the chord. 
____ It is interesting to find the peculiar structure 
_ of so important a part as the spinal chord re- 
oe in two species, which, with the excep- 
tion of the dermal spines, and their common 
_ charactersas Mammalia, differ in other respects 
as widely from one another, and occupy such 
distant places in their class. Can the short- 
_ hess of the solid chord, and the great length 
of the nerves within the spinal canal, have 
any physiological relation with the habit, 
common to both the placental and monotre- 
- Matous hedgehogs, of rolling the body into a 
_ ball when torpid or asleep, or when the tegu- 
mentary armour is employed in self-defence ?* 
_ The olfactory nerves are large in the Orni- 
thorhynchus (fig. 181, 1,1). The external root 
de Meckel, et que les déterminations de 1’anato- 
_ -miste Anglais doivent etre adoptées.”” — Voyage de 
Favorite, p- 166. 
ce 
' “ Cet échidné passait Ia majeure partie de son 
_ temps dans une espéce d’engourdissement, blotti, 
' enroulé 4 la maniére des hérissons.”— Voyage de la 
Favorite, p. 159. 
| VOL. III. 
MONOTREMATA. 
385 
is remarkable for its 
length and relative size: 
it arises from the poste- 
rior surface of the cere- 
bral hemisphere imme- 
diately behind the bige- 
minal body ; bends round 
the crus cerebri to the 
inferior surface; and is 
continued forward to join 
the internal root which 
rises from the base of 
the anterior lobes of the 
brain. 
In the Echidna the 
olfactory nerves may be 
described as enormous. 
The external root (fig. 
183, 1a) arises from 
nearly the whole anterior 
part of the natiform pro- 
tuberance, which extends 
its origin, as in the Or- 
nithorhynchus, to the pos- 
terior part of the hemi- 
sphere. The internal root 
(183, 1 6) is also very 
large: the lateral ventri- 
cle is prolonged forwards 
into the olfactory nerve, 
which would appear like 
a continuation of the en- 
tire hemisphere, were it 
not that it is overlapped 
by the anterior convo- 
lution. 
The extent and. com- 
plications of the olfactory 
cavity are proportionate 
in the two Monotremes 
to the size of their respec- 
tive nerves. 
The optic nerve (fig. 
181, 183, 2) is small in 
both Monotremes, in ac- 
cordance with the dimi- 
nutive size of the eye: 
the two nerves are joined 
by a transversely oblong 
chiasma. 
The eye is protected, 
in the Ornithorhynchus, 
by a cartilaginous plate 
continued from the upper 
rgb of the orbit, which 
eckel compares with 
the bony palpebral plates 
Brain and spinal chord, in the Crocodile. Both 
Echidna, Half natural Monotremes have a well 
size. ( Original.) developed membrana nic- 
titans: there are also an upper and a lower 
eyelid, each of which has its proper apertor 
muscle. 
In the Ornithorhynchus the sclerotic is carti- 
laginous, the cornea flabby, the retina very 
thick: there is no trace of pecten or marsu- 
pium: the lens is very small, two lines in 
vertical and transverse diameter, one line in 
antero-posterior diameter ; the mes 2 surface 
c 
