網頁圖片
PDF
ePub 版

dum" (Scarpa). The ganglionic system is closely connected with both the brain and spinal cord at its offset, and keeps up a communication with this last, through the whole of its course, by means of a white and a gray filament, which both pass between each ganglion and the anterior root of each spinal nerve. Thus the series of nerves and ganglia which send out branches to every part connected with respiration, nutrition, and circulation, are united by interchange of fibres with the spinal cord, and are thus connected with the brain.

The

8. The sympathetic system may be considered as the chief agent in the maintenance of animal life: for the maintenance of life depends on nutrition, and nutrition consists in the constant assimilation of fresh substance to supply the place of what is thrown off in the continual state of movement and change which constitutes what we term good health. analysis of animal bodies gives about four elementary substances (hydrogen, oxygen, carbon, and nitrogen), which are found in like manner to compose the air that we breathe and the food that we eat: but no human art has succeeded in compounding from them the smallest particle of organized matter, and we remain in great measure ignorant of the exact nature of the changes which convert food and air into the texture of the body. All that we can know, therefore, is, that the sympathetic system is the immediate instrument of effecting these changes, and that by some yet undiscovered properties of its nerves and ganglia, inanimate matter is made to share the life of the part to which it is assimilated. It is remarkable that, unlike all other nerves, those connected with the system we are describing, are neither susceptible of sensation so long as they continue in a healthy state, nor do they require an effort

The organs

of the mind to keep them in action. supplied by the sympathetic nerves are equally unlike the other members of the body in properties and in structure. For the limbs are disabled for a time by fatigue after long-continued exertion, whereas the heart, lungs, &c., whose vital action is sustained by this system, never require rest, although always in exercise. As it is essential to our existence that the operation of these organs should be unintermitting, it is most fortunate for us that they so rarely excite our notice; for we should never enjoy a moment's repose, were it necessary to keep up the circulation, respiration, &c., by a constant attention to them. And though this system exacts so little from our intellectual powers, rejects the control of our will, and rarely disturbs us by exciting a sensation, it nevertheless does strongly sympathize with our bodily and mental feelings. The heart, whose unwearied and unfelt movements are the result of its influence, throbs uneasily during the period of anxious and fearful expectation, and so forcible is the impulse given by powerful emotion, as sometimes to rupture the parts by a rush of blood. I have already touched on the connection of nerves by which this is effected, and it must be noticed again when I come to treat of instinctive emotions.

9. Such then is the apparatus of mere organic life. But this life requires support and defence in all but the very lowest division of the animal kingdom. The crypto-neura are, indeed, without exception, inhabitants of fluids; they therefore depend for subsistence on the casual nutriment that may floated towards them: their bodies, too, like plants, may be mutilated to a great extent, and yet preserve their vitality, as they are capable of reproducing a

be

severed part: but it is not so with the higher orders of animals: with them dismemberment is fatal, or at best irreparable. These also have to select or to seek their food, and must be warned against the approach of danger: a further apparatus of nerves is accordingly provided, by which they can take cognizance of external objects, and these nerves are usually termed the nerves of the senses. I shall not stay to inquire in how large a degree the inferior orders of animals possess them; in the higher they consist of smell, sight, hearing, taste, touch, and perhaps,-for on this point physiologists are not wholly satisfied,of general sensation.* For all but the two last named,

* A curious case recorded by the late Dr. J. Cheyne seems to favor the opinion that there may be a set of fibres conveying to the brain a sense of general sensation independent of the sense of touch. "We know an instance," says he, "of a remarkable delusion, arising from complete loss of feeling in the left side of the body, caused by an attack of palsy, which first originated, and then fatally terminated in apoplexy. In the morning the individual maintained that he had two left arms, and when we tried to convince him that he was under a misconception, he promptly offered to produce the supplementary arm. 'There it is,' said he, patting his left shoulder with his right hand. Well then,' it was asked, 'where is the other?'-On which, turning round his head with great alacrity to show it, he seemed much disappointed when he could discover but one arm, vehemently declaring that there were two, in the night."" Cheyne's Essays, p. 60. Here there must have been general sensation in the arm, or the patient would not have felt that he had an arm at all-but when in the night he felt but could not see that he had an arm, and on touching the surface of the palsied limb with the other hand, was sensible of no impression, he naturally supposed the real arm to be existing behind or beside the dead substance which he touched. Between sleeping and waking even in health we do not always reason, and here probably the reasoning power was somewhat disturbed by the lesion of the brain. If there should be a sense of this kind, it would account for the fact that pain is felt in palsied limbs which are insensible to touch; as well as for those cases of insanity or idiocy where the sense of touch remains, but that of heat, or the pain ensuing from a burn, is lost.

if two they be, a system of nerves within the skull, and in direct communication with the brain, is provided; whereas, the sense of touch being distributed over the whole body, is conveyed to the common centre of sensation by an immense number of nervous filaments, which either plunge into the spinal cord through small openings in the bone provided for them, and thus find their way to the brain, or are immediately connected with it.

10. Thus far I have described the machinery of life and sensation only, but it is further necessary that the living sentient animal should have the means of preserving his existence, of seeking pleasure, and of avoiding pain. This is accordingly provided for by another set of nerves, the nerves of voluntary motion. The operation of these nerves however is, in respect of direction, opposite to that of the nerves of sensation. It is by means of the latter that constant communications from all parts of the body to the brain are carried on. The nerves of motion on the contrary issue from the brain, and convey its mandates to whatever part it would control. This constant interchange somewhat resembles what is carried on between the provinces and the capital by the mail-trains. The nerve of sensation, like the train which conveys letters to the capital, receives continual contributions from the tracts which it passes through, until the whole, compressed into the smallest compass, is delivered at the central post-office: and in like manner the nerve of motion, like the outtrain, keeps sending forth its district mails at each successive station, until the most distant one is delivered at the terminus.

11. Thus we have three distinct systems of nervous mechanism in the living body, each dependent on the other, namely,

1. The unconscious involuntary nerves of life; II. The conductors of external and internal feelings to the brain;

III. The conveyers of volition from the brain to the organs fitted for action;

which are respectively termed the sympathetic, the sensitive, and the motor nerves.

66

12. I have already described the Sympathetic System (7) as a series of ganglia with connecting nerves, whose office it is to supply the nervous energy by which the functions of circulation, secretion, &c., are unconsciously carried on. The composition of these nerves differs considerably from that of the spino-cerebral system, being formed chiefly of a gray gelatinous fibre, not found in any great abundance elsewhere. These fibres seem to form an intermediate substance between the vesicular, and fibrous nervous matter; (4.) for they contain among them numerous cell nuclei, some situated in the centre of the fibre, others adhering to either edge, and frequently exhibiting distinct nucleoli."-"The mode of connection of the gelatinous fibres with the elements of the nervous centres," say the authors of the work from which I have already quoted, "is, as yet, quite unknown. They are found in considerable numbers in what are called the roots of the Sympathetic, or the communications of that nerve with the spinal nerves; and it has been supposed by Valentin that they are continuous with certain elements of the vesicular nervous matter." "*_That vital power by which the common functions of nutrition and reproduction are carried on, has been termed by Professor Liebig," vegetative life," and in the acknowledged obscurity which hangs over the modus operandi of

* Todd and Bowman's Phys., vol. i. p. 212.

« 上一頁繼續 »