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 vita.' The medulla oblongata is broad and de pressed : its inferior surface exhibits the corpora pyramidalia (fig. 181, a), the corpora olivaria (ff), 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 (b), (figured by Meckel as the pons Varolii); and anterior to those is the true nodus encephali' (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 gliuid 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 avoird upoise, 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 incline forwards (fig. 182) ; anterior to these is a larger convolution bent upon itself at a right angle, one erns running transversely ; the other longitudinally, and forming the inner boundary of the anterior halfof each hemisphere: this convolution was not divided by a transverse anfractuosity, as in the figure in the ' Voyage de In 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 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 arachnid 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 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 pone Varolii (c) presents, as in the Omi 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, fig. 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 sells turcica. The corpus mam 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 fig. 184, two elongated convex bodies are exposed, as in the marsupial brain : the posterior and largest (h) 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 pocarnpi 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 dum and fifth ventricle are entirely absent.