A phrase has been used suggesting that a ganglion presides over a part of the body, re ceiving impressions from it by one set of fibres and controlling the changes in it by impulses discharged along other fibres. It has also been mentioned that one ganglion is connected with others, so that the changes going on in one part may be related to those going on in another, that the harmonious working of all the parts may be maintained. This is specially well seen in certain animals, such as worms, where the body of the animal obviously consists of a series of segments, arranged longitudinally, each seg• went in its main features resembling the others. Each segment has its own ganglion or ganglia and related nerves. Still further, in animals of a certain degree of organization there is an obvious symmetry between one side of the body and the other, so that the body can be divided into two lateral halves markedly resembling one another ; and this symmetry is reproduced in the nervous system. Instead of one ganglion presiding over one segment of the hotly with its related nerves, there is a pair of ganglia side by side, each with its related nerves, and each presiding over its lateral half of the seg ment, the two being closely connected by fibres I passing between. Fig. 91 illus trates such paired ganglia, in which the two ganglia are close together, the fibres connecting them not being, therefore, ob vious, but in other animals they are widely separated, and the commissural fibres, as the connecting strands are called, are quite distinct.
Here we must repeat that however ganglia are multiplied by the requirements of increas ing complexity of structure, and however numerous and large become the nerve strands connecting them together, the type and essence of the whole structure is that already de scribed. The type is a nerve cell with a nerve-fibre carry ing an impression inwards, which impression provokes some change in the cell, as a result of which an impulse is discharged out wards by another nerve-fibre, resulting in some change in muscular fibre, or blood-vessel, or gland, or other structure. The fibre carrying the impression inwards is called an afferent or sensory fibre, and the fibre conveying the im pression outwards, efferent or motor fibre, and the process is known as a reflex action. Refer to page 132, where this is more fully explained. We may now add that while an afferent fibre carries impulses only inwards to the cell, and the efferent only outwards to the muscle, vessel, gland, &e., the two fibres may, and indeed very commonly are, bound up in the same strand or nerve-trunk. The nerve-trunk in such a case contains both motor and sensory fibres, conveys that is to say, both outwards and in wards. This is called a mixed nerve.
Now let us see what stage we have arrived at in our effort to understand the building up of a nervous system. The simplest conceivable nerve mechanism is a nerve-cell with an affer ent and efferent fibre. Nerve-cells are grouped
into ganglia, nerve-fibres into strands. As com plexity of structure increases, ganglia, and the nerves connecting them and the various organs and parts of the body, multiply, but they are more or less symmetrically arranged, till a double chain of ganglia with connecting strands is reached. As complexity goes on increasing, the ganglia come to be more closely placed, till they and their connecting strands become fused, so that at last a continuous cord is developed, nerves radiating from it.
Fig. 92 illustrates the nervous systems of a frog and a bird. Let the brain be neglected in each case. In the frog the bilateral arrange ment is shown by the groove partially dividing the spinal cord into two lateral halves, while the segmented arrangement is just indicated by the regularity with which the nerves come off from each side. Here, that is to say, the chain of paired ganglia with their commissural fibres have become fused into a continuous cord, the construction of which it would be impossible to understand but for the previous study of more elementary forms.
The similarity between these and the spinal cord of man is too obvious to need comment (Fig. 93). The bilateral symmetry of the human body is quite apparent, and if the development cord of man and the higher animals consists essentially of paired ganglia arranged longi tudinally, the two ganglia of each pair being closely bound together and each pair closely bound with every other by commissural fibres, each pair presiding over and regu lating the changes occurring in its own seg ment of the body (the process going on being of the nature of a reflex action), one ganglion of each pair looking after its own lateral half of the segment, the connection of the ganglion with its lateral half-segment being effected by means of afferent and efferent fibres, the whole being fused and bound together into one apparently con tinuous structure. And this is the truth, though not the whole truth. The view just stated receives corroboration from the mere naked-eye inspection of the human spinal cord. It is not of uniform thickness throughout its length, but is markedly enlarged at the upper and lower ends. It is at the level of these enlargements that the nerves come off which supply the upper and lower limbs, and a micro scopical examination of sections of the cord at these levels shows groups of nerve-cells parti cularly large and numerous. That is to say, to meet the requirements of the limbs there occurs a considerable multiplication of ganglia in the segments of the cord with which they are con nected. Then a cross section of the cord appears to the naked eye to consist of two absolutely identical lateral halves, held together about the middle by a narrow connecting fibrous bridge.