The parts of the living body possess also some powers which result merely from their organization, and have been denoted by physiologists under the epi thet of vis mortua. Thus they admit of extension beyond the natural state from extraneous impulse, and of contraction when that impulse ceases to operate. This extensibility and contractility are independent of life, and are terminated only by death. The stretching of mus cles by moving a limb, the extension of the skin over a tumour, its retraction when divided, &c. are examples of these powers. They have been confounded by some physiologists with the insensible organic contractility.
A muscle exhibits all the powers now enumerated. It contracts, in obedience to the will, by its animal contractility ; from the application of stimuli, by its or ganic sensible contractility. Its nutrition and growth show the existence of orga nic insensible contractility ; and its re traction on a section exemplifies the con tractility of organization. The internal organs of the body have only the three last powers, and the white organs (car tilage, tendon, ligament, &c.) only the two last. While, therefore, the two first properties exist only in certain parts, the latter are found in all. Hence the orga nic insensible contractility may be select ed as the general character of all living parts ; and the contractility of organiza tion as the common attribute of all living or dead parts that are organically con structed.
As for porosity, divisibility, elasticity, and the other properties which living bo dies have in common with inanimate mat ter ; they hardly deserve mention here, because they are never exerted in their whole extent, or in their genuine purity, if we may use that phrase. Their results are always affected by the influence of the vital powers, which constantly mo dify those effects which seem to flow most directly from physical, mechanical, or chemical causes.
Digestion. Is a function common to all animals, by which foreign substances, in troduced into their bodies, and submitted to the action of certain organs, change their qualities and form a new compound, fit for the purposes of nourishment and growth. Animals alone are provided with digestive organs ; all, from man to the polype, have an alimentary cavity, and its existence is, therefore, an essential character of animals. The loss which the body sustains in performing the various actions that take place in the living ani mal machine, is supplied by means of the food. Hunger and thirst admonish us of the wants of our frame, and the pleasures of the palate are a no less strong induce ment to the procuring and taking of food.
The cause of hunger has been placed in the mutual attrition of the rugs: of the empty stomach ; in the irritation produc ed by the gastric juice, &c. Perhaps it may be derived more justly from a sym pathy between the stomach and the body at large. For when, in diseases of the
pylorus, the food cannot be transmitted into the, intestines, and does not there fore enter the system, great hunger is experienced, even although the stomach may be filled. Much depends on habit, and on the operations of mental cau ses: hunger is felt at the usual periods of our repasts ; and, if it be not then re moved by eating, will often cease spon taneously. The man of letters, absorbed in meditation, often forgets the natural wants of his body. Whatever diminishes the sensibility of the stomach makes hun ger more tolerable. Thus, the Indian and Turkish fanatics (Mollahs and Fa kirs) are said to support their long fasts by the habitual use of opium. Thirst seems to consist more in a very trouble some dryness of the fauces and (esopha gus, and in a peculiar irritation of these parts from the admixture of acrid, and particularly saline matters, with the food. The necessity of obeying both these calls varies according to the age, constitution, and particularly the habits of individuals yet we may state, on the whole, that a healthy adult could not abstain from food for a whole day without bringing on con siderable weakness ; and that this absti nence could not be continued to the eighth day without the most imminent risk of life. Continued abstinence dimi nishes the weight of the body to a de gree which becomes sensible in twenty four hours, causes absorption of fat, great prostration of strength, increased sensi bility with watchfulness, and a most pain ful dragging at the epigastric region. Hunger is more speedily fatal in propor tion to the youth and strength of the individual. Thus, the wretched father, whose dreadful history is immortalised by Dante, shut with his children in a dun geon, perished last, on the eighth day of confinement, after witnessing the death of his four sons, amid the convulsions of rage and cries of despair. We meet with a large collection of examples of long abstinence in the great work of Haller ; but they do not seem to possess, in every instance, the requisite authenti city. Many of the subjects were weak and delicate women, living in a state of almost complete inaction, where the powers of lite, almost extinct, were only evinced by a very low pulse and respira tion repeated at long intervals. They might be compared to hybernating ani mals, where the waste, occasioned by the functions of active life, does not take place, and where consequently the usual supplies cannot be needed. Although the admonitions of thirst are very imperious, yet drink does not seem so nentcsary to life and health as solid food. The mouse. quail, parrot, and several other warm blooded animals never drink, and in stances have been knows in the human subject. Thirst always becomes greater when any watery secretions are much augmented, as in dropsy, and particularly in diabetes.