The possession of a nervous system is not essential to life, since in the whole vegetable kingdom, as well as in the lower animal organizations, multitudes of living forms are to be seen, which, although unprovided with nerves, are yet perfectly capable of preserving their independence and of holding their own in the struggle for existence; but wherever it is found, owing to its wonderful sensitiveness to impressions, it fulfils the important purposes of bringing the animal into relation with the outer world, of enabling it to respond to those impressions by inducing muscular movements which effect either local or general change of place or form, and finally of linking together, as with a subtle net-work, the most remote organs of the body, enabling each part to co-operate with the rest for the general good, and uniting or integrating them into a common whole.
Originally the nervous system is composed of a soft living mobile substance, termed " protoplasm", from which all parts of the body are formed; and it is only by degrees that it acquires its special endowment, that of generating nerve energy, which, like other forms of force, is subject to laws of its own, and can either be stored up, liberated, intensified, or exhausted under appropriate conditions.
In the horse, as in all the higher animals, the nervous system presents two parts for examination, one of which, and by far the larger, is named the cerebro-spinal, the other the sympathetic system.
The cerebro-spinal system is adapted to respond to various kinds of impressions made upon the organs of sense, as the eye, ear, skin, tongue, and nose, to conduct those impressions through cords, which are termed nerves, to central organs represented by the spinal cord, medulla oblongata, cerebellum, and brain, giving rise in the first instance to responsive movements of protection or defence, and then successively, as they affect higher and higher centres, to sensations of sight, hearing, touch, taste, and smell, and finally to ideas, emotions, and intellectual operations.
The sensory impressions or stimuli thus carried to the nerve centres may not be followed by any visible effect, but in most instances, especially in animals, impulses, which we may conceive to be waves or rapidly-propagated chemical or molecular changes, start, or are liberated from the centres, which travel along similarly constituted cords or nerves, and are conducted to muscles or to glands, exciting the former to contraction and the latter to secretion. The cords conducting impressions from the organs of sense to the centres are named " afferent" nerves, whilst those which transmit impulses from the centres to the muscles and glands are termed " efferent" nerves. The terms "sensory", "motor", and "secretory" nerves are, however, most commonly used.
The sympathetic system of nerves, sometimes named the nervous system of organic life, is destined to regulate the supply of blood to each organ of the body in accordance with its requirements, keeping the blood-vessels contracted when the organ is at rest, but permitting them to dilate under the influence of other nerves when in it is the active discharge of its functions. Thus in the fasting state the stomach is pale and quiescent, but in full digestion it is rosy, and performs active movements. The sympathetic system thus, by its action on the vascular system, indirectly but powerfully influences movement and secretion. It is composed of a series of knots or swellings, termed " ganglia ", united to one another by nerve cords. The more important ganglia form a chain lying on either side of the spinal column, and extending through nearly its whole length (fig. 168). Other ganglia belonging to this system, termed "collateral ganglia", are widely distributed in the body, and give off branches which accompany the blood-vessels, and finally enter the muscular tissue in their walls. The two systems of cerebro-spinal and sympathetic nerves have intimate relations with each other. Their structure is very similar.
The sympathetic system consists of numerous nerve cords ami ganglia distributed over the body, and destined to control and regulate the organs of vegetative life. The main trunks of this system are two in number, one running on either side of the vertebral column, extending from the head backward as far as the tail.
a, a, a, Ganglion Cells. i, b, Their Nuclei, c, c, c, Axis Fibres, d, d, Spiral Fibres.
Each in its course has upon it a number of small round or ovoid bodies termed ganglia. These consist of a covering of connective tissue, from which small septa pass into the interior. The spaces thus formed in the organs are filled in with small cells, some of which are round, while others have proceeding from them small fibres or poles, by which they are connected with nerve-tubes, which go to (afferent) and come from the ganglia (efferent). Some nerve-fibres also pass through the ganglia, and in doing so are brought into contact with the cells.
Fig. 168. - Diagram of the Ganglia of the Sympathetic System of Nerves.
1 Superior Cervical Ganglion. 2,2- Cervical Sympathetic Cord. 3 Middle Cervical Ganglion. 4 Inferior Cervical Ganglion. 5 Cervical Portion of Sympathetic. 6, 6 Dorsal Sympathetic. 7 Lumbar Portion of Sympathetic. 8 Sacral Portion of Sympathetic. 9 Great Splanchnic Nerve. 10 Lesser Splanchnic Nerve. 11 Solar Ganglion. 12 Afferent Branches from Spinal Pairs. I3 Pelvic Plexus. 14 Branch to Pelvic Plexus. 15 Spermatic Plexus. 16 Posterior Mesenteric Plexus. 17 Branches from Posterior to Anterior Mesenteric Plexus. 18 Lutnbo-Aortic Plexus. 19 Superior (Esophageal Branch. 20 Inferior Oesophageal Branch. 21 Cardiac Nerves. 22 Branch to Pelvic Plexus. 23 Conjoined Cord of Pneumogastric and Sympathetic Nerves.
As the sympathetic chain runs along the side of the vertebrae, a small ganglion appears upon it, opposite to each intervertebral gap or hole, out of which the spinal nerves emerge.
The spinal nerves on passing out of the spinal canal divide into an upper and a lower branch, and from each of the latter a few fibres proceed to the sympathetic ganglia and reinforce the sympathetic chain.
The different parts of the sympathetic cord are distinguished by terms indicating the region with which they are connected, hence the terms cervical or neck, the dorsal or back, the lumbar or loin, and the sacral or croup plexuses or nerves.