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called the cephalic or cerebral ganglion, which sends nerves to the organs of special sense, and which is regarded as the seat of volition and general sensation for the entire body. Following this is a pair of ganglia (3, s), the pedal or locomotory ganglia, which supply the muscular mantle and its foot-like expansion, and which regulate the movement of these organs. Finally, another ganglion (4), situated at the posterior part of the body, sends nerves to the branchiæ or gills, and is termed the branchial or respiratory ganglion. All these nervous centres are connected by commissures with the central or cerebral ganglion, and may therefore act either independently or in association with each other, by means of these connecting fibres. In the third type of animals, again, viz., the articulata, the general plan of structure of the body is different from the foregoing, and the nervous system is accordingly modified to correspond with it. In these animals, the body is composed of a number of rings or sections, which are articulated with each other in linear series. A very good example of this type may be found in the common centipede, or scolopendra. Here the body is composed of twenty-two successive and nearly similar articulations, each of which has a pair of legs attached, and contains a portion of the glandular, respiratory, digestive and reproductive apparatuses. The animal, therefore, consists of a repetition of similar compound parts, arranged in a longitudinal chain or series. The only exceptions to this similarity are in the first and last articulations. The first is large, and contains the mouth; the last is small, and contains the anus. The first articulation, which is called the "head," is also furnished with eyes, with antennæ, and with a pair of jaws, or mandibles.

Fig. 125.

[graphic]

The nervous system of the centipede (Fig. 125), corresponding in structure with the above plan, consists of a linear series of nearly equal and similar ganglia arranged in pairs, situated upon the median line, along the ventral surface of the alimentary canal. Each pair of ganglia is connected with the integument and muscles of its own articulation by sensitive and motor filaments: and with those which precede and follow by a double cord of longitudinal commissural fibres. In the first articu

NERVOUS SYSTEM OF CENTIPEDE.

lation, moreover, or the head, the ganglia are larger than elsewhere, and send nerves to the antennæ and to the organs of special sense. This pair is termed the cerebral ganglion, or the "brain."

A reflex action may take place, in these animals, through either one or all of the ganglia composing the nervous chain. An impression received by the integument of any part of the body may be transmitted inward to its own ganglion and thence reflected immediately outward, so as to produce a movement of the limbs belonging to that articulation alone; or it may be propagated, through the longitudinal commissures, forward or back, and produce simultaneous movements in several neighboring articulations; or, finally, it may be propagated quite up to the anterior pair of ganglia, or "brain," where its reception will be accompanied with consciousness, and a voluntary movement reflected back upon any or all of the limbs at once. The organs of special sense, also, communicate directly with the cerebral ganglia; and impressions conveyed through them may accordingly give rise to movements in any distant part of the body. In these animals the ventral ganglia, or those which simply stand as a medium of communication between the integument and the muscles, are nearly similar throughout; while the first pair, or those which receive the nerves of special sense, and which exercise a general controlling power over the rest of the nervous system, are distinguished from the remainder by a well-marked preponderance in size.

In the centipede it will be noticed that nearly all the organs and functions are distributed in an equal degree throughout the whole length of the body. The organs of special sense alone, with those of mastication and the functions of perception and volition, are confined to the head. The ganglia occupying this part are therefore the only ones which are distinguished by any external peculiarities; the remainder being nearly uniform both in size and activity. In some kinds of articulated animals, however, particular functions are concentrated, to a greater or less extent, in particular parts of the body; and the nervous ganglia which preside over them are modified in a corresponding manner. In the insects, for example, the body is divided into three distinct sections, viz: the head, containing the organs of prehension, mastication, tact and special sense; the chest, upon which are concentrated the organs of locomotion, the legs and wings; and the abdomen, containing the greater part of the alimentary canal, together with the glandular and generative organs. As the insects have a greater

amount of intelligence and activity, than the centipedes and other worm-like articulata, and as the organs of special sense are more perfect in them, the cerebral ganglia are also unusually developed, and are evidently composed of several pairs, connected by commissures so as to form a compound mass. As the organs of locomotion, furthermore, instead of being distributed, as in the centipede, throughout the entire length of the animal, are concentrated upon the chest, the locomotory ganglia also preponderate in size in this region of the body; while the ganglia which preside over the secretory and generative functions are situated together, in the cavity of the abdomen.

All the above parts, however, are connected, in the same manner as previously described, with the anterior or cerebral pair of ganglia. In all articulate animals, moreover, the general arrangement of the body is symmetrical. The right side is, for the most part, precisely like the left, as well in the internal organs as in the external covering and the locomotory appendages. The only marked variation between different parts of the body is in an antero-posterior direction; owing to different organs being concentrated, in some cases, in the head, chest, and abdomen.

Finally, in the vertebrate type of animals, comprising man, the quadrupeds, birds, reptiles and fish, the external parts of the body, together with the locomotory apparatus and the organs of special sense, are symmetrical, as in the articulata; but the internal organs, especially those concerned in the digestive and secretory functions, are unsymmetrical and irregular, as in the molluscs. The organs of respiration, however, are nearly symmetrical in the vertebrata, for the reason that the respiratory movements, upon which the function of these organs is immediately dependent, are performed by muscles belonging to the general locomotory apparatus. The nervous system of the vertebrata partakes, accordingly, of the structural arrangement of the organs under its control. That portion which presides over the locomotory, respiratory, sensitive, and intellectual functions forms a system by itself, called the cerebrospinal system. This system is arranged in a manner very similar to that of the articulata. It is composed of two equal and symmetrical halves, running along the median line of the body, the different parts of which are connected by transverse and longitudinal commissures. Its ganglia occupy the cavities of the cranium and the spinal canal, and send out their nerves through openings in the bony walls of these cavities.

The other portion of the nervous system of vertebrata is that which presides over the functions of vegetative life. It is called the ganglionic, or great sympathetic system. Its ganglia are situated anteriorly to the spinal column, in the visceral cavities of the body, and are connected, like the others, by transverse and longitudinal commissures. This part of the nervous system is symmetrical in the neck and thorax, but is unsymmetrical in the abdomen, where it attains its largest size and its most complete development.

The vertebrate animals, as a general rule, are very much superior to the other classes, in intelligence and activity, as well as in the variety and complicated character of their motions; while their nutritive or vegetative functions, on the other hand, are not particu larly well developed. Accordingly we find that in these animals the cerebro-spinal system of nerves preponderates very much, in importance and extent, over that of the great sympathetic. The quantity of nervous matter contained in the brain and spinal cord is, even in the lowest vertebrate animal, very much greater than that contained in the system of the great sympathetic; and this preponderance increases, in the higher classes, just in proportion to their superiority in intelligence, sensation, power of motion, and other functions of a purely animal character.

The spinal cord is very nearly alike in the different classes of vertebrate animals. It is a nearly cylindrical cord, running from one end of the spinal canal to the other, and connected at its anterior extremity with the ganglia of the brain. (Fig. 126.) It is divided, by an anterior and posterior median fissure, into two lateral halves, which still remain connected with each other by a central mass or commissure. Its inner portions are occupied by gray matter, which forms a continuous ganglionic chain, running

Fig. 126.

[graphic]

CEREBRO-SPINAL SYSTEM OF MAN.

from one extremity of the cord to -1. Cerebrum. 2. Cerebellum 3, 3, 3. Spinal

the other. Its outer portions are

cord and nerves. 4, 4. Brachial nerves. 5, 5. Sacral nerves.

composed of white substance, the filaments of which run for the most part in a longitudinal direction, connecting the different parts of the cord with each other, and the cord itself with the ganglia of the brain.

The spinal nerves are given off from the spinal cord at regular intervals, and in symmetrical pairs; one pair to each successive portion of the body. Their filaments are distributed to the integument and muscles of the corresponding regions. In serpents, where locomotion is performed by simple, alternate, lateral movements of the spinal column, the spinal cord and its nerves are of the same size throughout. But in the other vertebrate classes, where there exist special organs of locomotion, such as fore and hind legs, wings, and the like, the spinal cord is increased in size at the points where the nerves of these organs are given off; and the nerves themselves, which supply the limbs, are larger than those originating from other parts of the spinal cord. Thus, in the human subject (Fig. 126), the cervical nerves, which go to the arms, and the sacral nerves, which are distributed to the legs, are larger than the dorsal and lumbar nerves. They form also, by frequent inosculation, two remarkable plexuses, before entering their corresponding limbs, viz., the brachial plexus above, and the sacral plexus below. The cord itself, moreover, presents two enlargements at the point of origin of these nerves, viz., the cervical enlargement from which the brachial nerves (4, 4) are given off, and the lumbar enlargement from which the sacral nerves (5, 5) originate. If the spinal cord be examined in transverse section (Fig. 127), it will be seen that the gray matter in its central portion forms a double crescenticshaped mass, with the concavity of the crescents turned outward. These crescentic masses of gray matter, occupying the two lateral halves of the cord, are united with. each other by a transverse band of the same substance, which is called the gray commissure of the cord. Directly in front of this is a

Fig. 127.

[graphic]

Transverse Section of SPINAL CORD.-a, b. Spinal nerves of right and left side, showing their two roots. d. Origin of anterior root. e. Origin of posterior root. c. Ganglion of posterior root.

transverse band of white substance, connecting in a similar manner

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