BLOOD, MORBID CONDITIONS OF THE.—The nature and properties of blood in its normal condition having been considered in the foregoing article, we proceed to notice those changes to which it is liable in a state of disease.
That a fluid which is destined to receive and convey materials for the formation, increase, and repair of every structure in the animal frame, which carries away whatever is useless, and is brought into perpetual contact with the external atmosphere, should itself be subject to morbid alterations, is a notion so natural, so entirely in accordance with what might a priori be ex pected, that, independently of all reasoning, and antecedently to all proof, it has existed in the common belief of every age and of every nation.
To preserve a healthy state of the blood has accordingly ever been considered an object of primary importance. The greatest pains have been taken to maintain its purity, as well in the individual as the species ; not only in man, but in all those animals which he has domesticated for his use; and there is no belief more generally received than that which attributes the origin of many of the cutaneous eruptions, and of most of the cachectic diseases, to the degene racy and poverty of this vital stream.
When from this general and popular notion we advance to the more especial assumption that the origin of all diseases is to be found in the blood and other fluids ; when we classify these into hot and cold, moist and dry, or into blood, bile, black bile and phlegm, and attribute morbid changes and even natural dispositions to the prevalence of one or other of these supposed humours, we quit the belief of the people to follow theories far less tenable, invented at a period when authoritative assertions had the weight of proof, and when the dogmata of a philosopher were preferred to facts plainly re corded in the book of nature.
It would be out of place here to enter into a discussion of the merits of the humoral pa thology as compared with the various doctrines which have supplanted it, and to which it is not unlikely that in an improved form it may again succeed.
Under the triple relation of vital phenomena, intimate structure, and chemical composition, as Andral justly remarks, we can draw no definite line of demarcation between the blood and the solids. Physiologically speaking, we cannot conceive that of these two facts which form a single whole, the one can he modified without affecting the other. Since the blood nourishes the solids, they must necessarily be influenced by its state ; and since the solids furnish ma terials from which the blood is formed, and abstract materials by which it is decomposed, any alteration in the nature or quantity of these must necessarily have its influence on this fluid.
Suffice it then to observe that the further we extend our knowledge of pathology, the less shall we feel inclined to admit the exclusive claims either of fluidism or solidism, and the more shall we strengthen our belief that the animal structure is composed of parts, every one of which may not only partake of disease, but, under certain circumstances, become its cause.
Quitting, therefore, all unprofitable specu lations on this subject, we proceed at once to a detail of facts, and to such observations in elu cidation of them as occasion may suggest.
Blood may be excessive in quantity, thus constituting a state of plethora in which the circulating system is supplied more abundantly than is needed for the due performance of the functions of nutrition and secretion. A ten dency to accumulation in the capillaries and in the different internal organs is induced, and con gestion with its consequences, or actual rupture of the bloodvessels, is the result. Drowsiness, vertigo, headache, epilepsy, apoplexy, mark this state as existing in the head ; dyspncea, and a livid or purple hue of the skin, as affecting the lungs ; palpitation and irregular action with syncope mark the ineffectual struggle of the heart to propel its contents. Haemorrhages from the mucous membranes of the nose, the lungs, or the intestines, are often the consequence of congestion in the vessels which ramify on their surface; while indigestion, torpor, and biliary redundancy, are connected with a plethoric condition of the abdominal viscera. Although the existence of such a state, as deducible from the symptoms just enumerated, as well as from the effect which depletion has in removing them, admits of no doubt, it has, nevertheless, not been made the subject of direct proof. The proportion which the circulating blood, even in a healthy animal, bears to its total weight has not been, and, perhaps, cannot be ascertained with precision. Haller collects together many authorities at variance with each other on this point, and at length comes to the conclusion, " Neque dissimulandum est, obiter hem et wage definiri. Infinita enim procul dubio in ratione sanguinis ad reliquam corporis molem varietas est." f Fat men and animals have less blood than lean, old than young ; and yet plethora is oftener found in the former than the latter, obviously on account of the mechanical im pediment which the encumbered tissue or the rigid fibre offers to the circulation.