The effects of bleeding are to lessen the quantity of blood for a time, and to impair its quality. The first effect is too obvious to require proof; the second is not so obvious, but not less certain. After the loss of blood, absorption of liquid takes place immediately, in order to supply the deficiency; so that the blood-vessels are soon as full as they were previously to the bleeding. Nature has provided this resource to prevent emptiness of the circulation, and the want of that equable pressure which the blood everywhere exerts, and without which the systemic actions could not be supported in their integrity. But it is not fully elaborated blood that thus supplies the loss. The vessels take up such liquids as they can find. Water is absorbed from the alimentary canal, and probably from the. air; and the various saline and albuminous fluids existing in the system are eagerly appropriated. The water, salts, and albumen lost are, therefore, soon replaced; and the requisite proportion of fibrin is quickly elaborated; but the red corpuscles, which require a longer time, and a more complex process for their development, remain much longer deficient. When the loss of blood has been great, whether from one copious bleeding, or several successive bleedings, this deficiency is very obvious in the paleness of the patient, and the translucency of his tissues; and the anemic condition continues often for a considerable time. Therefore, though the immediate effect of bleeding is to diminish the mass of the blood, the loss in this respect is soon repaired; and the permanent effect is a depreciation of the quality of that fluid, which is rendered more watery, and less able to supply influence and nutriment to the systemic functions.

The immediate phenomena attendant on the loss of blood are a diminution in the fulness and force of the pulse, paleness of the surface, and reduction of the temperature of the body. As the loss proceeds, the pulse becomes more and more feeble, till it can scarcely be felt; the respiratory movements diminish proportionably; the lips, face, and general surface become increasingly pale, and the skin cool; feelings of languor, nausea, muscular weakness, giddiness, mental confusion, and faintness come on; and at last complete syncope takes place, with a temporary suspension of all the obvious vital processes, from which the patient gradually recovers, if the blood ceases to flow.

Syncope. The amount of loss necessary to produce syncope varies exceedingly in different persons in health, and in the same persons in disease, according to the character of the affection, and the special tissue affected. Thus, it is usually much greater in those of sanguine temperament, and the plethoric, than in the nervous and feeble; and in diseases attended with pressure on the brain, acute inflammation, or a sthenic condition of fever, than in asthenic affections with anaemia, or a depraved condition of the blood, as in typhoid fevers. in the former set of diseases more is required, as a general rule, than in health, in the latter less. The loss of blood is usually better borne in the inflammations of the serous and fibrous membranes, than in those of the mucous membranes and the skin. its degree of effect is often influenced by idiosyncrasy. Stout and apparently vigorous individuals, in consequence of peculiarity of constitution, sometimes faint under a small bleeding; while others, seemingly feeble, bear a great loss without the same result. imagination is not without influence. I knew a gentleman of stout habit of body, whom the sight of his own blood caused to become extremely faint.

The posture of the body has great effect; that position most resisting syncope which occasions the greatest pressure on the brain. Hence, much more blood can be taken from a person lying down than sitting, and sitting than standing; and important practical rules are deducible from this fact. it is probable that the greater muscular effort used in the two latter positions may aid in the effect; as, by drawing blood to the muscles, it lessens the force with which it is directed to the head.

The mode in which the blood is drawn has considerable influence; less being required when it is taken rapidly through a large orifice, than when slowly through a small one. The reason of this probably is the sudden diminution of the normal pressure upon the cerebral centres in the former case; while in the latter the diminution is gradual, so that the brain has a better opportunity of accommodating itself to the new circumstances.

"According to Dr. Marshall Hall, the mean loss of blood required to produce symptoms of commencing syncope, in the sitting or erect posture, is, in health, 15 ounces; in congestion of the brain, from 40 to 50 ounces; in inflammation of the serous, synovial, or fibrous membranes, from 30 to 40 ounces; in parenchymatous inflammations, 30 ounces; in mucous and cutaneous inflammations, 16 ounces; while in eruptive fevers it is only from 12 to 14 ounces; in delirium tremens, 10 to 12; in accidents before inflammation has taken place, 8 to 10; and in chlorosis, 8." But these are mere approximations, and liable to great diversity. in relation to the natives of this country, I am quite confident that the 15 ounces, stated by Dr. Hall as the mean in health, is considerably below the actual average.

The syncope from loss of blood is not unfrequently attended with convulsive symptoms; but these, like other nervous disorders, are probably quite as liable to occur from a depressed as from an excited state of the cerebral centres. They do not, therefore, in any degree contradict the idea, that the influence of bleeding is essentially sedative. But certain phenomena do not unfrequently occur after the loss of blood, which are clearly the result of abnormal excitement, and which I now propose to explain.

Secondary Excitation. All the functions require, for their due performance, a certain amount of blood of a certain quality. if there be deficiency in either of these respects, they necessarily suffer. To obviate injury from this cause, a relation between all parts of the body and the nervous centres has been established through connecting nerves, of such a nature, that a deficient supply of blood for the functions causes an impression to be conveyed from their seats up to their respective centres, by which the latter are excited into action, and, if the impression be strong, are not unfrequently much disturbed. As a legitimate and intended result of this excitement, an influence is transmitted from the centres to the heart, and probably to the respiratory organs, by which the former is stimulated to increased action, so as to send an additional supply of their essential pabulum to the functions, while by the latter the blood is duly prepared for its office. Now blood, which has been impaired in quality by bleeding, is unable, in the ordinary quantity, to supply the demands of the functions unless they also are proportionably lowered; and consequently, if this should not happen, the heart is stimulated, through the nervous centres, to supply, by a greater amount of blood, the deficiency in its quality. But the nervous centres being often over-excited by these demands upon them, act irregularly in the performance of their duties; and consequently signs of nervous disorder are evinced. Hence, in the condition of system induced by bleeding, especially if very copious or frequently repeated, there is often great disturbance of function, which must be regarded as the indirect effects of the loss of blood. Violent palpitation of the heart, excessive frequency of pulse, dyspnoea, headache, neuralgic pains in various parts of the body, wakefulness, sometimes even convulsions and delirium, are among the evidences of this nervous disorder.

But it is important to bear in mind that, in order that this indirect excitement from bleeding should take place, there must be a want of accordance between the performance of the functions and the diminished quality of the blood. if the former continue as in health; if, for example, the patient use as much muscular exertion, eat as much, think as much, even feel as much as under ordinary circumstances, the above symptoms must be experienced in a degree proportionate to the impoverishment of the blood; and, if the functions are called into unusual activity, whether by the will or other cause; if the patient run, ascend heights, lift heavy burdens, attempt to think deeply, or exert his mind strongly in any way, or be thrown into strong emotion by any cause, the deficiency in the supply of blood is of course more strongly felt, a more powerfully irritant influence is sent to the nervous centres; and the disorders, consequent on their disturbance, are often experienced to an extreme, and sometimes alarming, or even fatal extent.

if, however, care be taken that the various functions shall be reduced in accordance with the reduced state of the blood; if the patient be kept at rest, the amount of food diminished, the mind made to participate in the repose of the body, and nothing allowed to disturb the calm of the passions, the purely sedative effects of the bleeding will be experienced, without the secondary excitements and disorders above mentioned.

It will be perceived, from this course of reasoning, how important it is, in employing this remedial measure, to take care that its intended effects be not contravened by subsequent disturbing influences Another highly important practical inference is that, by carrying the remedy too far, we may induce an excitable state of system, which shall render the patient liable to extreme disorder, and especially to excessive action of the heart, ending possibly in serious disease of that organ. One of the greatest dangers of the excessive use of the lancet is the production of hypertrophy and dilatation of the heart; the former resulting, in accordance with a well-known physiological law, from a too powerful and incessant exertion of the organ, the latter from a softness and yieldingness of the tissue, consequent on the anemic state of the blood.

Bleeding should very rarely be carried to complete syncope, and never when used simply as an antiphlogistic measure. The subsequent reaction, under the operation of causes above explained, may very much interfere with the ends proposed, and do more harm than the occurrence of syncope can possibly do good. The only circumstance which would justify the urging of the remedy to this extent, in the practice of medicine at least, is the existence of a spasm, as of the glottis, for example, which unless relaxed must prove fatal, and which there are no other means at command capable of relaxing. In surgery, the measure has sometimes been resorted to, in order to procure complete relaxation of the muscles in cases of obstinate dislocation, which have resisted all ordinary measures. But since the discovery of etherization, I doubt whether bleeding to syncope can now be required in any case.