altogether voluntary, inasmuch as we only swallow when we choose, yet it is not so in reality. For what the Will does, is to carry back the particle to be swallowed, by a movement of the tongue, so as to bring it into contact with the membrane lining the pharynx ; and this contact serves to call the muscles of the pharynx into automatic action, whereby the particle is grasped and carried downwards into the gullet. It has several times happened that a feather, with which the back of the mouth was being tickled in order to excite vomiting (another form of reflex action), having been carried down a little too far, has been thus grasped by the pharyngeal muscles, and drawn out of the fingers of the operator. In sucking, again, there is a combination of respiratory movements, producing the vacuum which draws forth the milk, with the movements by which it is swallowed; and the whole combination is a purely reflex action, performed by the instrumentality of a ganglionic centre which forms no part of the Brain proper, and called-forth by the contact, either of the nipple of the mother, or of something which produces the like impression, with the lips of the offspring (§ 69).—This last act is sometimes spoken of as instinctive, and has been even taken as a type of that class of operations; and in the broad sense of the term Instinct, it may doubtless be so regarded. But, in common with the ordinary and extraordinary movements of respiration, with swallowing, and, with many other actions that are immediately concerned in the maintenance of the Organic functions, it may be executed unconsciously; requiring nothing for its performance but an automatic Mechanism of nerves and muscles, which, in its normal state, responds as precisely to the stimulus made upon it, as the Locomotive steam-engine does to the directing actions of its driver. The actions to which it seems preferable to limit the term instinctive, are those to which the prompting is given by sensations. These are not less "reflex" than the preceding in their essential nature, being the automatic responses given by the

Nervous mechanism to the impressions made upon it, in virtue of its original or acquired endowments; but the Nerve-centres concerned in them being of a higher order, their reflex activity cannot be called forth without affecting the consciousness of the Animal that executes them (§§ 57, 77, 78).

49. In ascending through the Molluscous series, we find the Nervous system increasing in complexity, in accordance with the increasing complexity of the general organization; the addition of new organs of special Sensation, and of new parts to be moved by Muscles, involving the addition of new ganglionic centres, whose functions are respectively adapted to these purposes. The possession of a distinct head, in which are located the organs of Vision, the rudimentary organs of Hearing, and the organs (if any such exist) of Smell and Taste, constitutes the distinction between the two primary divisions of the series, the cephalous and the acephalous; the Snail and Whelk being typical examples of the former, the Oyster and Cockle of the latter. In the Cephalous Mollusks, we always find a pair of ganglia situated in the head; which pair, termed the cephalic ganglia, is really made up of several distinct ganglionic centres, and is connected by cords that pass round the œsophagus, with other ganglia disposed in various parts of the trunk. Still, generally speaking, the Nervous system bears but a small proportion to the whole mass of the body; and the ganglia which minister to its general movements, are often small in proportion to those which serve some special purpose, such as the actions of Respiration. This is what we should expect from the general inertness of the character of these animals (typified by the term sluggish), and from the small amount of Muscular structure which they possess.

50. Again, we find no other multiplication of similar centres, than a doubling on the two sides of the body; excepting in a few cases in which the organs they supply are correspondingly multiplied,as in the arms of the Cuttle-fish, which are furnished with great

numbers of contractile suckers, every one possessing a ganglion of its own. Here we can trace very clearly the distinction between the reflex actions of each individual sucker, depending upon the powers of its own ganglion; and the actions prompted by Sensation, which are called forth through its connection with the Cephalic ganglia. For the Nerve-trunk which proceeds to each arm may be distinctly divided into two tracts; one containing the ganglia which appertain to the suckers and are connected with them by distinct filaments; whilst the other consists of fibres that form a direct communication between these and the Cephalic ganglia. Thus each sucker has a separate relation with a ganglion of its own, whilst all are alike connected with the Cephalic ganglia, and are placed under their control; and we see the results of this arrangement, in the mode in which the contractile power of the suckers may be called into operation. When the animal embraces any substance with its arm (being directed to this action by its Sight or some other sensation), it can bring all the suckers simultaneously to bear upon it; evidently by a determinate impulse transmitted along the connecting cords that proceed from the Cephalic ganglia to the ganglia of the suckers. On the other hand, any individual sucker may be made to contract and attach itself, by placing a substance in contact with it alone; and this action will take place equally well when the arm is separated from the body, or even in a small piece of the arm when recently severed from the rest, thus proving that when it is directly excited by an impression made upon itself, it is a reflex act, quite independent of the Cephalic ganglia, not involving Sensation, and taking place through the medium of its own ganglion alone.*

51. In the Articulated series, on the other hand, in which the

* A very curious example of the independent activity of the gangliated cord in the arm of the Cuttle-fish, and of its similarity, both in structure and action, to the ventral cord of Articulata, is presented in the detached Hectocotylus-arm of the male of the Argonaut (Paper-Nautilus), which, when first discovered, was mistaken for a Worm.

Fig. 7.

Locomotive apparatus is highly developed, and its actions are of the most energetic kind, we find the Nervous system almost entirely subservient to this function. In its usual form, it consists of a chain of ganglia connected by a double cord; commencing in the head, and passing backwards through the body (Fig. 7). The ganglia, though they usually appear single, are really double; being composed of two equal halves closely united on the median line. In general we find a ganglion in each segment, giving-off nerves to the muscles of the legs, as in Insects, Centipedes, &c.; or to the muscles that move the rings of the body when no extremities are developed, as in the Leech, Worm, &c. In the lower Vermiform (or worm-like) tribes, especially in the marine species, the number of segments is frequently very great, amounting even to several hundreds; and the number of ganglia increases in the same proportion. But whatever be their degree of multiplication, they seem but repetitions of one another; the functions of each segment being the same with those of the rest. The cephalic ganglia, however, are always larger and more important; they are connected with the organs of special Sense; and they evidently possess a power of directing and controlling the movements of the entire body, whilst the power of each ganglion of the trunk is for the most part confined to its own segment.

GANGLIATED.

NERVOUS CORD

52. The Cephalic ganglia lie above the mouth, in the immediate neighbourhood of the eyes, with OF CENTIPEDE. which they are connected by nerve-trunks. And from the constancy of the relation between the size of these ganglia and the development of the Visual organs, it cannot be doubted that they are to be regarded as essentially optic ganglia, though also containing the

ganglionic centres of the nerves of other Senses, altogether constituting the Sensorium.-These Cephalic ganglia are connected with the ganglion of the first segment of the trunk, by a band on either side; and the pair of bands, with the ganglia above and below, form a ring through which the oesophagus passes, so that the chain of ganglia comes to lie nearer the lower or ventral surface, beneath the alimentary canal, instead of just beneath the dorsal surface, above the alimentary canal, which is the position of the Spinal cord of Vertebrata. Hence the longitudinal gangliated chain of Articulated animals is often distinguished as the ventral cord.

53. A marked difference is observable in the arrangement of the ganglia of the Ventral cord, according as the act of Locomotion is performed by muscles uniformly repeated through the successive segments of the body, as in the crawling of the Maggot or Caterpillar; or by the muscles of special appendages, attached to particular segments, as in the perfect. Insect. In the former case, the ganglionic chain is uniform throughout; whilst in the latter, the ganglia of the thorax, with which are connected the nerves that supply the legs and wings, are greatly increased in size, whilst those of the abdomen, the segments of which no longer take any share in the act of locomotion, are proportionally reduced. The change from one condition to the other takes place during the metamorphosis.—When the structure of the Ventral cord is more particularly inquired into, it is found to consist of two distinct tracts; one of which, composed of nerve-fibres only, passes backwards from the Cephalic ganglia over the surface of all the ganglia of the trunk whilst the other includes the collections of nerve-cells which constitute ganglia. Hence every part of the body has two sets of nervous connections; one with the ganglion of its own segment, and another with the Cephalic ganglia. Each of the ganglia of the Ventral cord ministers to the reflex actions of its own segment, and, to a certain extent also, to those of other segments: for by