For the due performance of the function of nutrition certain conditions are necessary, a which the most important are-1, a right state and composition of the blood, from which the materials of nutrition are derived; 2, a regular and not far distant supply of such blood; 3, a certain influence of the nervous system; and 4, a natural state of the part to be nourished.
1. There must be a certain adaptation peculiar to each individual between the blood and the tissues. Such an adaptation is determined in its first formation, and is main tained in the concurrent development and increase of both blood and tissues. This main tenance of the sameness of the blood is well illustrated by the action of vaccine matter. By the insertion of the most minute portion of the virus into the system, the blood undergoes an alteration which, although it must be inconceivably slight, is maintained for several years; for even very long after a successful vaccination, a second insertion of the virus may have no effect, because the new blood' formed after the vaccination con tinues to be made similar to the blood as altered by the vaccine matter. So, in all prob ability, are maintained the morbid states of the blood which exist in syphilis and many other chronic diseases; the blood once inoculated, retaining for years the taint which it once received. The power of assimilation which the blood exercisesin these cases is exactly comparable with that of maintenance by nutrition in the tissues; and evidence of the adaptation between the blood and the tissues, and of the delicacy of the adjust ment by which it is maintained, is afforded by time phenomena of symmetrical diseases (especially of the skin and bones), in which. in consequence of some morbid condition of the blood, a change of structure affects in an exactly similar way the precisely corres ponding parts on the two sides of the body, and no other parts of even the same tissue. These phenomena (of which numerous examples are given in two papers by Dr. W. Budd and Mr. Paget in the lath vol. of the Medico-chirargical Transactions) can only be explained on the assumption—lst, of the complete and peculiar. identity in composition in corresponding parts of opposite sides of the body; and 2dly, of so precise and com plete an adaptation between the 'pod and the several parts of each tissue, that a morbid material being present in' the bloggi, may destroy its 'fitness- for the nutrition of one or two portions of a tissue, without affecting its fitness for the maintenance of the other portions of the same tissue. If, then, the blood can be fit for the maintenance of one
part, and unfit for the maintenance of another part of the same tissue (as the skin or bone), how precise must be that adaptation of the blood to the whole body, by which in health it is always capable of maintaining all the different parts of the numerous organs and tissues in a state of integrity.
2. The necessity of an adequate supply of appropriate blood in or near the part to be nourished, is shown in the frequent examples of atrophy of parts to which too little blood is sent, of mortification when the supply of blood is entirely cut off, and of defec tive nutrition when the blood is stagnant in a part. The blood-vessels themselves lake no share in the process, except as the carriers of the nutritive matter; and provided they tome so near that the latter may pass by imbibition, it is comparatively unimportant Whether they ramify within the substance of the tissue, or (as in the case of the non vaecular tissues, such as the epidermis, cornea, etc.) are distributed only over its surface or border.
3. Numerous cases of various kinds might be readily adduced to prove that a certain influence of the nervous system is essential to healthy nutrition. Injuries of the spinal cord are not unfrequently followed by mortification of portions of the paralyzed parts; and both experiments and clinical cases show that the repair of injuries takes place less completely in parts paralyzed by lesion of the spinal cord than in ordinary eases.' Divi sion of the trunk of the t•ifacial nerve has been followed by incomplete nutrition of the corresponding side of the face, and ulceration of the cornea is a frequent consequence of the operation.
• 4. The fourth condition is so obvious as to require no special illustration.
For furthur information on this most important department of physiology, the reader is referred to Mr. Paget's Surgical Pathology, or to his original lectures on nutrition, hypertrophy, and atrophy (published in vol. 39 of The Medical Gazette) or to the chapter on " Nutrition and Growth," in Kirke's Handbook of Physiology, which contains an excel lent abstract of Mr. Paget's views, and to which we are indebted for the greater part of this article.