MONOTREMATA. 



383 



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 hippocampus 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 part of the ventricle, and is confluent 

 with the inferior and external parietes of that 

 cavity. The corpus stnatum is long and nar- 

 row : the thalamus opticus small, and is united 

 with its fellow by a soft commissure, which 

 rises to the same level, whereby they appear 

 to 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- 

 gether absent in the ' testes,' which thus form 

 a single small tubercle. It is in the condition 

 of these parts, recognized, but too briefly no- 

 ticed by Meckel, that the brain of the Orni- 

 tliorhynchm deviates most essentially from the 

 Marsupialia, and offers the most direct step in 

 the descent to the Oviparous type. 



The cerebellum is moderately large, highest 

 in the middle, but with small lateral append- 

 ages: the median or vermiform part is traversed 

 by transverse furrows ; and its vertical section 

 exhibits an ' arbor vitae.' 



The medulla oblongata is broad and de- 

 pressed : its inferior surface exhibits the corpora 

 pyramidalia (fig. 181, a), the corpora olivaria 

 ('), which expand as they advance forwards, 

 apparently in relation to the immense size of 

 the trigeminal nerve. Their anterior extremi- 

 ties are crossed by large trapezoid bodies (6), 

 (figured by Meckel as the pons Varolii) ; and 

 anterior to those is the true ' nodus er.cephali' 

 (c), which is narrow, in correspondence with 

 the small lateral lobes of the cerebellum ; and 

 from this there emerges on each side a large gan- 

 glioid body (c'\ from which the trigeminal nerve 

 (5) arises. The under surface of the medulla 

 oblongata is traversed by a deep median lon- 

 gitudinal groove. 



The brain of the Echidna is relatively larger, 

 and its external surface is complicated by 



convolutions.* 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. 



i 



Brain of the Echidna , right hemisphere dissected. 

 (Original.) 



incline forwards (fig. 182); anterior to these 

 is a larger convolution bent upon itself at a 

 right angle, one crus running transversely ; 

 the other longitudinally, and forming the inner 

 boundary of the anterior half of 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- 

 responding hemisphere. The depth of the me- 

 dian fissure of the hemispheres is from five 

 to six lines: the hippocampal commissure (o) 

 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, torn. v. pi. 9, p. 161. 



