384 



MONOTREMATA. 



transparent; the membranous falx is less deep 

 than the osseous one of the Ornithorhynchus. 

 I found no bony plate between the hemi- 

 spheres in the Echidna. The arachnoid is 

 transparent, but relatively strong:. The cere- 

 bellum is traversed by several narrow trans- 

 verse anfractuosities, disposed as in fig. 182. 

 The vermiform or median lobe, as in the 

 Ornithorhynchus, is larger in proportion to 

 the lateral lobes than in the Marsupialia, and 

 the limits between them are much less dis- 

 tinctly defined. 



The base of the brain in the Echidna (fig. 

 183) is remarkable for the deep and wide 

 excavations at the under part of the anterior 



Fig. 183. 



Base of brain of Echidna, ( Original. ) 



lobes, forming the base of the enormous 

 olfactory nerves. The natiform protuberances 

 are unusually large. The medulla oblongata 

 is broad and flat, but contracted anteriorly to 

 an angle ; the pyramidal bodies (a) long and 

 narrow ; the olivary bodies (b) broad, but flat. 

 The pons Varolii (c) presents, as in the Orni- 

 thorhynchus, a low development proportion- 

 ally to the size of the brain : it is not raised 

 beyond the level of the under surface of the 

 medulla oblongata : it is of a triangular form 

 with the obtuse apex turned forwards : the 

 median longitudinal groove formed by the ba- 

 silar artery is well marked : the trapezoid bodies 

 are relatively narrower than in the Ornitho- 

 rhynchus. 



The pituitary gland (p, Jig. 183) is one line 

 and a half in length and one line in breadth : 

 its under surface adheres closely to the dura 

 mater of the sella turcica. The corpus mam- 



Fig. 184. 



millare is single, broad, and but little ele- 

 vated. 



The internal, superior, and posterior walls 

 of the lateral ventricle are from one line to two 

 lines in thickness ; the outer wall is between 

 two and three lines ; when the roof of the 

 ventricle is removed, as in Jig. 184, two 

 elongated convex bodies are exposed, as in 

 the marsupial brain : the posterior and 

 largest (A) is the hip- 

 pocampus major: the 

 anterior body (s) is 

 the corpus striatum. 

 The whole internal 

 wall of one ventricle 

 is quite disunited from 

 that of the opposite 

 hemisphere.* 



The contracted an- 

 terior parts of the hip- 

 pocampi are connected 

 together by the short 

 transverse commissure 

 above mentioned, 

 which is the sole re- 

 presentative of the cor- 

 pus callosum and for- 

 nix. The septum luci- Right lateral ventricle laid 

 dum and fifth ventricle P fOrSST' 



are entirely absent. 



The pia mater, which accompanies the ven- 

 tricular artery into the floor of the ventricle, 

 at the base of the hippocampus, spreads over 

 the optic thalami, and sends upwards a smooth 

 fold of membrane, one line and a half broad, 

 between the hippocampus and the corpus 

 striatum. The free margin of this fold is 

 slightly thickened by the choroid artery. 



It is necessary to remove the hippocampus 

 and posterior part of the hemisphere, in order 

 to bring into view the optic thalami and bige- 

 minal bodies. 



The optic thalami (fig. 182, /) and nates 

 appear as one convex body slightly contracted 

 laterally, and divided from each other by a sig- 

 moid linear fissure : the testes are only half the 

 breadth of the nates, and the median longitudinal 

 line of division, which is very faint in the larger 

 bodies, is not visible in the smaller and pos- 

 terior tubercle. The Echidna corresponds in 

 this characteristic modification with the Orni- 

 thorhynchus.f 



* The internal structure of the hemispheres of 

 the Echidna's brain is not described in the' Voyage 

 de la Favorite.' 



t MM. Eydoux and Laurent have thrown into 

 the following tabular form the published results 

 of the dissections of the brains of the Impla- 

 cental Mammalia as compared with Placental 

 Mammals and Birds. 



They add : " En indiquant ce resultat des ob- 

 servations de M. R. Owen, aux-quelles nous avons 



joint les observations de Meckel en les rectifiant, 

 nous devons faire remarquer que Meckel a cepen- 



