MONOTREMATA. 
called ‘quadrigemina.’ Well preserved speci- 
mens of Ornithorhynchus presented to me by Mr. 
Thomas Bell, surgeon R. N., in 1838, have ena- 
bled me to determine this question. There is 
neither corpus callosum nor septum lucidum 
in the Ornithorhynchus. 
The part described by Meckel as the corpus 
callosum corresponds with the fornix and 
hippocampal commissure, as it exists in the 
Marsupialia, excepting that the essential func- 
tion of the fornix, as a longitudinal commis- 
sure, uniting the Aippocampus major with the 
olfactory lobe of the same hemisphere, is more 
_ exclusively maintained in the Ornithorhynchus, 
in consequence of the smaller size of the trans- 
_ verse band of fibres uniting the opposite hip- 
-pocampi, and representing the first rudiment 
_ of the corpus callosum, as it appears in the 
_ development of the placental embryo. The 
_ thin internal and superior parietes of one 
lateral ventricle are wholly unconnected with 
_ those of the opposite ventricle. 
__ Meckel makes no mention of the fornix or 
hippocampus major: the latter forms a large 
pyramidal prominence at the outer and pos- 
terior of the ventricle, and is confluent 
_ with the inferior and external parietes of that 
-eavity. The corpus striatum is long and nar- 
_ row: the thalamus opticus small, and is united 
"with its fellow by a soft commissure, which 
es to the same level, whereby they appear 
form a continuous body. The anterior com- 
_ Missure is very large, as in the Marsupials. 
The posterior bigeminal body is much 
smaller than the anterior, and the trans- 
verse depression which divides them is very 
feebly marked: the longitudinal groove is 
‘equally feeble on the ‘ nates,’ and is alto- 
er absent in the ‘ testes,’ which thus form 
le small tubercle. It is in the condition 
ese parts, recognized, but too briefly no- 
d by Meckel, that the brain of the Orni- 
hynchus deviates most essentially from the 
arsupialia, and offers the most direct step in 
descent to the Oviparous type. 
e cerebellum is modeiately large, highest 
the middle, but with small lateral append- 
: the median or vermiform part is traversed 
transverse furrows; and its vertical section 
_ exhibits an ¢ arbor vite.’ 
_- The medulla oblongata is broad and de- 
essed : its inferior surface exhibits the corpora 
nidalia (fig. 181, a), the corpora olivaria 
), which expand as they advance forwards, 
arently in relation to the immense size of 
trigeminal nerve. Their anterior extremi- 
are crossed by large trapezoid bodies (6), 
ured by Meckel as the pons Varolii); and 
rior to those is the true ‘ nodus encephali’ 
which is narrow, in correspondence with 
small lateral lobes of the cerebellum; and 
n this there emerges on each side a large gan- 
id body (c’), from which the trigeminal nerve 
arises. The under surface of the medulla 
Ongata is traversed by a deep median lon- 
dinal groove. 
he brain of the Echidna is relatively larger, 
and its external surface is complicated by 
responding hemisphere. 
383 
conyolutions.* It weighs twelve drachms and 
thirty grains avoirdupoise, and bears a proportion 
to the weight of the body as 1 to 50. The 
cerebral hemispheres conceal the bigeminal 
bodies, but do not extend over the cerebellum. 
The broad posterior part of each hemisphere 
is disposed in three nearly parallel transverse 
convolutions, the outer extremities of which 
Fig. 182. 
Brain of the Echidna, right hemisphere dissected. 
( Original. ) 
incline forwards (fig. 182); anterior to these 
is a larger convolution bent upon itself ata 
right angle, one crus running transversely ; 
the other longitudinally, and forming the inner 
boundary ofthe anterior halfof each hemisphere : 
this convolution was not divided by a transverse 
anfractuosity, as in the figure in the ‘ Voyage 
de la Favorite,’ loc. cit. On the outside of 
the longitudinal convolution there are two or 
three oblique folds which converge towards 
the contracted anterior part of the brain, or 
descend to its under surface: besides these 
principal and more constant convolutions there 
are a few smaller and less regular ones at the 
lateral and inferior parts of the hemispheres, 
especially on the great natiform protuberances. 
The principal anfractuosities sink more than 
a line’s depth into the substance of the he- 
misphere: the posterior convolutions are con- 
tinued upon the median surface of the he- 
misphere, and interlock with those of the cor- 
The depth of the me- 
dian fissure of the hemispheres is from five 
to six lines: the hippocampal commissure (0). 
one line and a half in antero-posterior diameter 
is seen at the bottom of the fissure which 
divides the hemispheres. 
The dura mater in the Echidna is thin and 
* See the figures and description of the external 
characters of the brain of the Echidna, given by 
MM. Eydoux and Laurent in the ‘ Voyage de la 
Favorite,’ 8vo. 1839, tom. v. pl. 9, p. 161. 
