In the highest class of animal life the brain is at once the seat of intelligence and sensibility, and the origin of the nervous system. In the lower animals intelligence and sensibility decrease exactly in proportion to the deviation of their nervous system from this high standard. The forms of the nervous system are more and more degraded as the animals sink in the scale of being, till at last creatures are found in which nerves have only been discovered with the microscope; others apparently have none, consequently they have little or no sensibility.
The brain and the spinal cord enclosed in the vertebræ of the backbone form a nervous system, which in the vertebrated creation is equal to all the contingencies and powers of these animated beings, but is beyond all comparison most perfect in the human race. The brain alone is the seat of consciousness, for the spinal cord, though intimately connected with it, and of a similar ‘mysterious albuminous electric pulp,’ appears to have no relation to the faculties of perception and thought, yet it is essential to the continuance of life. It is a distinct nervous centre which generates muscular energy in man and animals corresponding to external impressions, but without sensation, and is entirely independent of the will; the vegetative functions of respiration, the contractions of the heart, circulation of the blood, and digestion, are carried on under every circumstance, even during sleep. The reason of their being independent of sensation and the will is, that the nerves in the organs performing these functions never reach the brain, which is the seat of intelligence and sensation, but they form what is called the reflex system; for any impressions made upon them are carried to the upper part of the spinal cord alone, and are reflected back again to the muscles of the heart, lungs, &c., which, by their contractions, produce these involuntary motions. For instance, the flow of blood into the cavities of the heart while dilating, acts upon the nerves, and these excite a rhythmical movement in the muscular fibres of the heart. For there is a vital contractility in muscular tissue which is one of the most universal attributes of living beings, and is probably the sole cause of motion in the lowest grades of life, and the movements produced by it in the higher grades are in all cases the most directly connected with the vegetative functions. The involuntary reflex system of nerves constitutes the chief locomotive power in a number of the lower animals; but it forms a continually decreasing portion of the whole nervous system in proportion as animals rise in the scale of life, till in man its very existence has been overlooked. If the spinal cord were destroyed, instant death would be the consequence; whereas infants born without brain have sucked and lived for a day or two.
There are numerous actions, especially among the lower animals, as little under the influence of the will or intelligence as the reflex nerves, which nevertheless depend upon sensation for their excitement. The sensation may call the muscular apparatus into action without any exertion of reason or will, in such a manner as to produce actions as directly and obviously adapted to the well-being of the individual as the reflex system. For example, a grain of dust irritates the nostrils, and involuntarily excites the complicated muscular movements concerned in the act of sneezing. This class of actions, which is called sensori-motor, or consensual, includes most of the purely instinctive motions of the lower animals, which, being prompted by sensations, cannot be assigned to the reflex group.
Purely emotional movements are nearly allied to the preceding. Sensation excites a mental feeling, or impulse, which reacts upon the muscular system without calling either the will or the instinct into exercise. These emotional movements are often performed in opposition to the strongest efforts of the will, as when a sense of something ridiculous may excite irresistible laughter at an improper time. It is probable that the strong emotions exhibited by many of the lower animals, which have been ascribed to instinct, are referable to this group.[[1]]
The movements of such animals as have no nerves are merely owing to the vital contractility of muscular fibre.
In the highest province of animal life, which includes the mammalia, birds, reptiles, and fishes, the general structure of the nervous system consists of a double lobed brain, from whence a spinal cord proceeds, protected by articulated bones, which extend along the back of the animals, and from thence nerve-fibres extend to every part of the body. But in order to suit a great variety of forms, this system undergoes many modifications. In all the lower grades of life that have nerves, the system chiefly consists of small globular masses, or nuclei, of nervous matter, technically called ganglia, which are centres of nervous energy, each of which is endowed with its own peculiar properties; the nervous cords and filaments proceeding from them are merely organs of transmission. The arrangement of these centres of nerve-force is symmetrical, or unsymmetrical, according to the form of the animal.
In the lower portion of Articulated animals, such as insects, crustacea, annelids, worms, &c. &c., there is a double cord extending along the ventral side of the animal, united at equal intervals by double nerve-centres, or ganglia. These two cords diverge towards the upper end, surround the gullet, and unite again above that tube to form a distinct bilobed principal nerve-centre or brain. A third form of the nervous system is only a ring round the gullet; the points in it from whence the nerves radiate are swollen nerve-centres, or ganglia. Those on the sides and upper parts of the ring represent the brain, and supply the eyes, mouth, &c., with nerves: other centres, connected with the lower side of the ring, send nerves to the locomotive organs, viscera, and respiratory organs. In animals of a still lower grade there are single nuclei irregularly scattered, but in every case they are centres of energy from whence filaments are sent to the different parts of the creature. The last and lowest system consists of filamentous nerves, chiefly microscopic.
Intelligence, or the mental principle, in animals differs in degree, though not in kind, from that in the human race. It is higher in proportion as the nervous system, especially the brain, approximates in structure to that of man; but even in many of the lower orders may be traced the dawn of that intelligence which has made man supreme on earth. Every atom in the human frame, as well as in that of other animals, undergoes a periodical change by continual waste and renovation; but the same frame remains: the abode is changed, not the inhabitant. Yet it is generally assumed that the living principle of animals is extinguished when the abode finally crumbles into dust, a tacit acknowledgment of the doctrine of materialism; for it is assuming that the high intelligence, memory, affection, fidelity, and conscience of a dog, or elephant, depend upon a combination of the atoms of matter. To suppose that the vital spark is evanescent, while there is every reason to believe that the atoms of matter are imperishable, is admitting the superiority of matter over mind: an assumption altogether at variance with the result of geological sequence; for Sir Charles Lyell observes, that ‘sensation, instinct, the intelligence of the higher mammalia bordering on reason, and lastly the improvable reason of man himself, presents us with a picture of the ever-increasing dominion of mind over matter.’
The physical structure of a vast number of animals has been investigated from such as are a mere microscopic speck to the highest grade of animal life; but very little is comparatively known of their intelligence and means of communication. We know not by what means a pointer and greyhound make an agreement to hunt together; nor how each dog is not only aware that his companion possesses a property which he has not, but that by their united talents they might accomplish their purpose, which is merely sport, for they never eat the game.[[2]] The undulations of the air and water are no doubt the means by which most animals communicate; but there is reason to believe that many inhabitants of the earth, air, and water are endowed with senses which we do not possess, and which we are consequently incapable of comprehending.