It must not be supposed that anything like a regular serial order or progression is to be observed in the development of the Brain among Mammals. In the higher types of lower orders it will often be found better developed than among the lower types of higher orders. Still if we compare the extremes of the class— that is, higher with lower Mammals-a great increase in the developmental complexity of the organ, or in type of Brain, as judged by the human standard, will become perfectly obvious.

The ratio of the weight of the Brain to the weight of the body, is subject to great variations, from different causes, so that a table of such ratios does not give any trustworthy information as to the relative development of the organ in different species of animals. We may be able to deduce some kind of rough average, sufficing to indicate its increasing development if we compare class with class-as Fishes with Birds, or Birds with Mammals -but in detail and for estimating the relative development of the Brain in different species, its indications are of little or no value. This may be illustrated by the following table in which some of these ratios are given :

It is, of course, obvious enough that the order indicated.

in the above series is one which does not correspond with

the Intelligence of the respective creatures; neither shall we find that it in the least degree harmonizes with the complexity of development to which the Brain attains.

One of the principal disturbing causes arises from the fact, that in animals of any given order, the bulk or weight of the Brain when passing from its smaller to its larger representatives, does not increase at all in the same proportion as the total body-weight of such animals. Some striking illustrations of this fact have been cited by Professor Owen.* Small and large representatives of the same order of animals are, in the subjoined list, bracketed together, in order to show how much greater is the ratio of brain-weight to body-weight in the diminutive forms.

In part explanation of these very interesting peculiarities, Prof. Owen advances the following hints. "The Brain," he says, "grows more rapidly than the body, and is larger in proportion thereto at birth than at full growth. . . . . So in the degree in which a species retains the immature character of dwarfishness, the brain is relatively larger than the body." This may be to some extent an explanation of the peculiarity above shown to exist; but there are, doubtless, other vital and mechanical reasons, why the bulk of the Brain should not increase quite proportionately with the bulk of the body.

We may now point out some of the more striking pecu* "Anat. of the Vertebrates," iii. p. 143.

liarities of the several parts of the Brain, as met with in different representatives of the great class of Quadrupeds.

The Medulla, the Cerebellum, and the pons Varolii, are so intimately related to one another, both structurally and functionally, that they may here be regarded as constituting one compound division of the Brain. There is nothing special to be said concerning the Medulla in Quadrupeds, except that the lateral projections, known as 'olivary bodies,' gradually become more developed (fig. 72, o). In many animals, a layer of fibres on each side, known as the corpus trapezoideum' (fig. 73), crosses these structures and partially hides them. In higher Quadrupeds, however, such transverse fibres cross the Medulla at a higher level or appear to be absent (fig. 74). Where this is the case the olivary bodies' are uncovered; and as they also become larger, they may form rounded prominences, one on each side of the Medulla. The above-mentioned 'corpora trapezoidea,' usually cross the Medulla at the level of the origin' of the auditory and occuli; em, corpus mammilfacial nerves. They are very distinct in the Lion, the Dog, and the Sheep.* pezoideum; pa, anterior The upper part of the Medulla is bridged above and closely embraced by a much thicker mass of fibres known as the pons Varolii, the development of which in different Mammals, is found to be strictly proportionate to the development of the lateral lobes of the Cerebellum.

FIG. 73.-Brain of Rabbit, under surface. (Solly, after

Leuret.) A, Olfactory lobe;

I, Lobe of the Hippocampus,

or 'processus pyriformis;' o, optic nerve; m, motor

lare; p c, crus cerebri; p", pons varolii; bt, corpus tra

* See Tiedemann's Icones Cerebri Simiarum,' Tab. III. and VII.

Where the Pons is well developed, the cerebral peduncles,' being more covered, appear to be curtailed in length (fig. 74, i, i).

The Cerebellum in Marsupials (fig. 68), still consists principally of the median lobe,' the surface of which is marked by deep transverse fissures, giving rise to a series of nearly parallel convolutions. Its lateral lobes' exist

FIG. 74.-Brain of Dolphin, under surface. (Owen, after Tiedemann.) a, Spinal cord; b, anterior pyramids; c, Pons Varolii; e, posterior inferior lobe of Cerebellum; f, anterior inferior lobe, g, amygdaloid lobe, and h, flocculus, all lobes of Cerebellum. i, i, Cerebral peduncles; p, corpus albicans; o, pituitary body; m, temporal lobe, and I, anterior lobe of Cerebrum. Olfactory bulbs absent; 2. optic nerves; 3, motor nerves of eyes (fourth nerve appears from above the Cerebellum, in front of g); 5, the trigeminus; 6, the sixth nerve; 7, the facial, and 8, the auditory nerves; 9, glosso-pharyngeal; 10, vagus; 11, spinal accessory; 12, hypoglossal; 13, first cervical

nerve.

merely as small appendages, and are thought by some anatomists to correspond in higher forms with certain accessory lobules, named flocculi.' Among Rodentia the lateral lobes show a marked increase in size, which is obvious in the Hare (fig. 76), and still more so in the Beaver (fig. 71) where these parts are distinctly larger

than the median lobe. In Solipedes, Ruminants, and Carnivores, the lateral lobes also begin to surpass the median in size. This increase is very notable among the latter in the Cat (fig. 79), and also in the Dog (fig. 80); but it is still more marked in many Cetacea, such as the Dolphin (fig. 74), and the Porpoise (fig. 77).

FIG. 75.-Brain of the Horse, upper aspect. (Owen.)

FIG. 76.-Brain of the Hare, upper aspect. (Spurzheim.) a, Olfactory lobes; b, Cerebral Hemispheres; d, Cerebellum; e, Medulla.

In some Solipedes and Carnivores, the Cerebellum, instead of consisting of broad and comparatively smooth lateral lobes, together with a narrower and much divided median portion (fig. 77), is, as Marshall says, very uneven upon its surface, apparently consisting of a

*Outlines of Physiology," vol. i. p. 414.