surfaces are shown in Figs. 5 and 6.-In the lateral strands of the Medulla Oblongata, close to the fourth ventricle, there is a pair of ganglionic centres (characterized by the presence of vesicular matter), in which the auditory nerve terminates; and these are sometimes developed as distinct ganglionic enlargements. Other separate ganglia, sometimes of considerable size, are very commonly found at the origin of the Par Vagum. It is curious to notice the very large comparative size of the Pineal gland (f), and of the Pituitary body (h), in this class; the functions of these organs are entirely unknown.

870. The analogy of the Optic lobes of Fishes to the Corpora Quadrigemina and Thalami Optici of the fully-formed brain of the higher Vertebrata, is not so complete as it is to certain parts which occupy their place at an earlier period. In the Human Embryo, at about the 6th week, the Encephalon consists of a series of vesicles arranged in a line

Human embryo of sixth week, enlarged about three times :-a, vesicle of corpora quadrigemina; b, vesicle of cerebral hemispheres; c, vesicle of thalami optici and third ventricle; d, vesicle for cerebellum and medulla oblongata; e, auditory vesicle; f, olfactory fossa; h, liver; ** caudal extremity.

with each other; of which those that represent the cerebrum (b, Fig. 153) are the smallest, whilst that which represents the cerebellum (d) is the largest. Between the cerebral and the cerebellic vesicles are two others (c, and a), of which the posterior one is the Optic ganglion, and answers to the Tubercula Quadrigemina; whilst the anterior contains the Third Ventricle, and corresponds in some degree to the Thalami Optici. This condition is precisely represented in the Lamprey; but in most Fishes, the optic ganglia, and the parts surrounding the third ventricle, form but one lobe; so that the third ventricle seems hollowed out of the cptic ganglia, as shown in Fig. 7, c, (Plate II.)

871. The Encephalon of Reptiles does not show any considerable advance in its general structure, above that of the higher Fishes. The Cerebral Hemispheres (Plate II. Figs. 8, 9, 10, b), are always much larger than the Olfactive and Optic ganglia; and they generally coverin the latter (c, c) in part, by their posterior extremities. The Cerebellum is almost invariably of small proportionate dimensions; and this is especially the case in the Frog, in which it does not even cover-in the fourth ventricle. This low development of the Cerebellum in Reptiles, is what might be anticipated from the general inertness of those animals, and the want of variety in their movements. The Spinal Cord is

still very large, in proportion to the nervous masses contained in the skull; and, as we shall hereafter see, its power of keeping up the movements of the body, after it has been cut off from all connexion with the brain, is very considerable.-We find that, in Reptiles, as in Fishes, the Spinal Cord may have a nearly uniform size from one extremity to the other, like the ventral cord of the lower Articulata; or it may present considerable enlargements at particular spots, like the ganglionic cord in the thoracic regions of Insects. This difference depends upon the degree of development of the special locomotive organs. Thus in the Eel and Serpent, whose movements are accomplished by the undulations of the entire trunk, and which are destitute of members, we find a uniform development of ganglionic matter in the spinal cord. On the other hand, in the Flying-fish, in which the pectoral fins or anterior extremities effect the greater part of the propulsion of the body, we find a great ganglionic enlargement of the Spinal cord, at the part in which the nerves of those members are connected: in the Frog, whose movements are chiefly effected by the posterior extremities, we find a similar enlargement at the roots of the crural nerves and in the Turtles and Lizards, the two pairs of whose members are nearly equal in function, and serve to effect the principal movements of the body, we find an anterior and posterior enlargement of the Spinal Cord, corresponding to the parts with which the nerves of these members are connected.

872. We find in Birds a considerable advance in the character of the Encephalon, towards that which it presents in Mammalia. The Cerebral Hemispheres (Plate II., Figs. 11, 12, 13, b) are greatly increased in size; and they cover-in, not merely the olfactory ganglia, but in great part also, the optic ganglia. The former are of comparatively small size; the organ of smell in Birds not being much developed. The latter are very large, in conformity with the acuteness of sight which is highly characteristic of the class. The cerebellum is of large size, as we should expect from the number and complexity of the muscular movements performed by animals of this class; but it is still undivided into hemispheres. The Spinal Cord is still of considerable size in comparison with the Encephalon; and it is much enlarged at the points whence the legs and wings originate. In the species which have the most energetic flight, such as the Swallow, the enlargement is the greatest where the nerves of the wings come off; but in those which, like the Ostrich, move principally by running on the ground, the posterior enlargement, from which the legs are supplied with nerves, is much the more considerable.

873. In the Mammalia we find the size and general development of the Encephalon presenting a gradual increase, as we ascend the series, from the non-placental Monotremes and Marsupials, towards Man. In the former, the Hemispheres exhibit no convolutions; and the great transverse commissure, or connecting band of fibrous structure, termed the corpus callosum, is deficient. As we rise through the true viviparous division of the class, we notice a gradually-increasing prolongation of the Cerebral Hemispheres backwards; so that first the optic ganglia, and then the cerebellum, are covered-in by them. The latter partly shows itself, however, in all but Man and the Quadrumana, when we

look at the brain from above downwards: as we see in the Encephalon of the Sheep (Plate II., Figs. 14, 15, d). The Cerebral hemispheres increase, not only in size; but also in complexity of structure, both external and internal. Their exterior, instead of remaining smooth, is marked by convolutions; which serve to extend very greatly the amount of surface over which blood-vessels can pass into the gray substance. Their internal structure becomes more complex, in the same proportion. as their size and the depth of their convolutions increase; and in Man all these conditions present themselves in a far higher degree than in any other animal. The number of commissural bands, connecting the two hemispheres with each other transversely, and uniting their anterior and posterior portions, is very greatly increased; and in fact, a large proportion of their mass is composed, in Man, and the higher Mammalia, of fibres of this character.-In proportion to the increase of the Cerebral hemispheres, there is a relative diminution in the size of the ganglia of special sense; but their dimensions, as compared with the entire bulk of the animal, are by no means reduced, but are even increased. The Olfactive ganglia (Fig. 14, a) are always readily discoverable; being separated from the remainder of the encephalic masses by a peduncle on each side. The Optic ganglia (Fig. 15, c), on the other hand, are so completely covered-in by the Hemispheres, that it is only when the latter are turned aside that we can discern them. They differ in external aspect from the optic ganglia of Birds and the lower Vertebrata; being divided by a transverse furrow into anterior and posterior eminences, whence they are known as the Corpora Quadrigemina. The Auditory ganglia are lodged in the substance of the Medulla Oblongata, forming the "gray nuclei" of the strands termed the "posterior pyramids;" and similar nuclei in the "restiform bodies" are the ganglionic centres of the Glosso-pharyngeal nerves, and perhaps minister to the sense of Taste. Besides these, however, are the two large bodies termed the Corpora Striata and Thalami Optici, which have been commonly considered as appendages of the Cerebrum, but which must undoubtedly be regarded as independent of it, and as themselves constituting ganglionic centres, whose development bears no constant proportion to that of the Cerebrum. From the peculiar relation presently to be described (§ 901), which these bodies bear on the one hand to the Spinal Cord, and on the other to the rest of the Encephalon, there seems strong reason to believe that they together constitute the ganglionic centre of the sense of Touch, and of the motions which are automatically prompted by it. The Cerebellum is chiefly remarkable for the development of its lateral parts or hemispheres, and for the intricate arrangement of the gray and white matter in them (Fig. 15, d); the central portion, sometimes called the vermiform process, is relatively less developed than in the lower Vertebrata, where it forms the entire organ.-The Spinal Cord is much reduced in size, when compared with other parts of the nervous centres; the motions of the animals of this class being more dependent upon their will or guided by their sensations; and the simply reflex actions bearing a much smaller proportion to the rest. The development of ganglionic enlargements, in accordance with the presence or absence of high locomotive powers in the extremities, follows the same rule as in the preceding classes.

3. Functions of the Spinal Cord and its Nerves.

874. In commencing our more detailed examination into the functions of the different parts of the Nervous system in Vertebrated animals, it seems best to commence with the Spinal Cord; this being the portion whose presence is most essential to the continuance of life. As already mentioned, Infants are sometimes born without any Cerebrum or Cerebellum; and such have existed for several hours or even days, breathing, crying, sucking, and performing various other movements. The Cerebrum and Cerebellum have been experimentally removed from Birds and young Mammalia, thus reducing these beings to a similar condition; and all their vital operations have, nevertheless, been so regularly performed, as to enable them to live for weeks, or even months. In the Amphioxus, as already remarked, we have an example of a completely-formed adult animal, in which no rudiment of a Cerebrum or Cerebellum can be detected. And in ordinary profound sleep, or in apoplexy, the functions of these organs are so completely suspended, that the animal is, in all essential particulars, in the same condition for a time as if destitute of them. It is possible, indeed, to reduce a vertebrated animal to the condition (so far as its nervous system is concerned) of an Ascidian Mollusc (§ 850); for it may continue to exist for some time, when not merely the Cerebrum and Cerebellum have been removed from above, but when nearly the whole Spinal Cord has been removed from below,-that part only of the latter being left, which is the centre of the respiratory actions, and which corresponds to the single ganglion of the Tunicata. On the other hand, no animal can exist by its Encephalon alone, the Spinal Cord being destroyed or removed; for the reflex actions of the latter are so essential to the continuance of its respiration, and consequently of its circulation, that if they be suspended (by the destruction of the portion of the cord which is concerned in them), all the organic functions must soon cease.

875. Although the Spinal Cord was formerly regarded as little else than a bundle of nerves proceeding from the Brain, yet its true rank, as a distinct centre of nervous power, is now universally admitted. That the actions prompted by it, when these do not originate in one of the higher centres, are of a purely reflex nature,-consisting in the excitement of muscular movements in respondence to external impres sions, without the necessary intervention of sensation,-appears to be a necessary inference from the facts that have been brought to light by experiment and observation. Experiments on the nature of this function are best made upon cold-blooded animals; as their general functions are less disturbed by the effects of severe injuries of the nervous system, than are those of Birds and Mammals. When the Cerebrum has been removed, or its functions have been suspended by a severe blow upon the head, a variety of motions may be excited by their appropriate stimuli. Thus, if the edge of the eyelid be touched with a straw, the lid immediately closes. If a candle be brought near the eye, the pupil contracts. If liquid be poured into the mouth, or a solid substance be pushed within the grasp of the muscles of deglutition, it is swallowed.

If the foot be pinched, or burned with a lighted taper, it is withdrawn ; and (if the animal experimented on be a Frog) the animal will leap away, as if to escape from the source of irritation. If the cloaca be irritated with a probe, the hind-legs will endeavor to push it away.

876. Now the performance of these, as well as of other movements, many of them most remarkably adapted to an evident purpose, might be supposed to indicate, that sensations are called up by the impressions; and that the animal can not only feel, but can voluntarily direct its movements, so as to get rid of the irritation which annoys it. But such an inference would be inconsistent with other facts.-In the first place, the motions performed by an animal under such circumstances are never spontaneous, but are always excited by a stimulus of some kind. Thus, a decapitated Frog, after the first violent convulsive movements occasioned by the operation have passed away, remains at rest until it is touched: and then the leg, or its whole body may be thrown into sudden action, which immediately subsides again. In the same manner, the act of swallowing is not performed, except when it is excited by the contact of food or liquid; and even the respiratory movements, spontaneous as they seem to be, would not continue, unless they were continually re-excited by the presence of venous blood in the vessels. These movements are necessarily linked with the stimulus that excites them; that is, the same stimulus will always produce the same movement, when the condition of the body is the same. it is evident, that the judgment and will are not concerned in producing them; and that the adaptiveness of the movements is no proof of the existence of consciousness and discrimination in the being that executes them, the adaptation being made for the being, by the peculiar structure of its nervous apparatus, which causes a certain movement to be executed in respondence to a given impression,—not by it. An animal thus circumstanced may be not unaptly compared to an automaton; in which particular movements, adapted to produce a given effect, are produced by touching certain springs. Here the adaptation was in the mind of the maker or designer of the automaton; and so it evidently is, in regard to the reflex or consensual movements of animals, as well as with respect to the various operations of their nutritive system, over which they have no control, yet which concur most admirably to a common end.

Hence

877. Again, we find that such movements may be performed, not only when the Brain has been removed, the Spinal cord remaining entire, but also when the Spinal cord has been itself cut across, so as to be divided into two or more portions, each of them completely isolated from each other, and from other parts of the nervous centres. Thus, if the head of a Frog be cut off, and its spinal cord be divided in the middle of the back, so that its fore-legs remain connected with the upper part, and its hind-legs with the lower, each pair of members may be excited to movement by a stimulus applied to itself; but the two pairs will not exhibit any consentaneous motions, as they will do when the spinal cord is undivided. Or, if the Spinal cord be cut across, without the removal of the Brain, the lower limbs may be excited to movement, by an appropriate stimulus, though they are completely paralysed