The following is a table, shewing the actual and relative lengths of the cerebral hemispheres and the cerebellum in the Mammalia:— On cutting into its substance in many of these animals, the appearance of the arbor vim is more or less distinct, similar to the human cerebellum. On its inferior surface is situated its great commissure, the pons Varolii, which first makes its appearance in this class of ani mals, and, with the exception of the transverse fibres forming it being thinner and fewer in number, particularly in those lower orders of Mammalia where the cerebral hemispheres were small, it presents but little differences from the same part in the human adult brain. This latter remark will equally apply to the fourth ventricle, which has been an object of consi derable interest, and which, from being at first an open exposed cavity, is now shut in and concealed.
[On the peculiarities of the brains of the implacental class of Maromalia, see the ar ticles AIARSUPIALIA and AIONOTRESIATA.1 On reviewing these statements of the nervous system in the Mammalia, we observe that the brain now preponderates greatly in bulk over the spinal marrow ; this latter is also shorter, and terminates by a true cauda equina. The first cerebral mass has now acquired its maxi mum of developement as regards size ; the two portions of which it is composed are united by a large commissure ; their exterior surface is convoluted. The second cerebral mass is divi ded into two pairs of ganglia, in which the cavities are obliterated. The third cerebral mass has lateral hemispheres developed, strim and convolutions on their exterior surface, and an important commissure, the pons Varolii, on its inferior surface.
flaying thus completed the investigations proposed at the commencement of this, paper, it may not in conclusion be without interest and utility to take a very rapid review of the development of the nervous system in the five large groups of animals in the system of ar mngement, as follows:— a. The nervous system (perhaps) first exists in a molecular form ; that is, it is made up of globules dispersed throughout the homogeneous texture of the animal, as in the Acrita, the lower Entozoa, &c.
b. This nervous matter arranged in a lon gitudinal direction forms filaments. The di rection which they assume is that of a ray, or nerve, and a central point, or ganglion ; these latter communicate with each other by commis sures, which unite them in the form of a ring. This ring is situated around the oral orifice of the animal; it takes the name of te primary nervous ring ; and from it issue filaments in a radiated manner, as in the F..chinoderrnata.
c. This oral primary nervous ring becomes more complicated in itself; ganglions are first developed on its lateral and inferior portions, from which nerves off in a longitudinal direction, as in rslower BIollusca, and secondly on its superior surface, as in the higher animals of this class : this superior g,anglion is at first proportionally small, as in the Gasteropoda, but afterwards large, and sometimes very large, as in the Cephalopoda. It is the analogue of the tubercula quadrigemina of the higher animals.
d. This primary nervous ring, in its most. highly developed form, now becomes repeated several times in the body of the animal; first, in an undetermined number, as in the Helmin thoid Articulata; secondly. in a determined number, as in the Entomoid Articulata. These nervous ring,s are united by longitudinal com missures, and the most anterior one always has a highly developed ganglion on its superior surface. The uniting commissures possess two distinct nervous tracts ; re.spiratory and syrnpathetic nerves exist, as in the Insecta.
e. These primary nervous rings are now become ganglia (brain); the uniting commis sures are become primary nervous rings (spinal' marrow). First, the ganglia and their commis sures are nearly equally developed, and are horizontal, as in the lower Vertebra% ; secondly, the ganglionic formation predominates, aud its direction, with regard to the comrnissures, be comes more that of a right angle, as in the higher Vertebrata; thirdly, the predominance of the ganglionic formation is very much in creased, and its relative direction is that of a complete right angle, as in the human species.
(John Anderson.)