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Rh complex by the circumstance that the living cell exhibits various morphological differentiations—above all, the differentiation in protoplasm and nucleus. Again, a transformation of energy is inseparably connected with metabolism. Along with food and oxygen potential chemical energy is continually being introduced into the cell, to be accumulated in the biogen molecules, and at their decomposition transformed into kinetic energy, which finds an outlet in the various manifestations of energy in the cell—motion, heat, and so forth. In the light of this hypothesis the operations of stimuli also become comprehensible. Seeing that there is an initial tendency to the occurrence of certain definite chemical processes, which are associated with the reconstruction and decomposition of biogen molecules, various stimuli will either further or hinder the course of this metabolic series. A cell which is exposed to no outward disturbance, and which continues always in the unvarying medium provided by an exact sufficiency of food, will be in “metabolic equilibrium”—that is to say, its assimilation and its dissimulation will be equal (A = D). When, however, the influence of external stimuli is brought to bear upon them—that is to say, any change in their environing vital conditions—A and D will either be altered in similar proportion, or their mutual equilibrium will be disturbed. In the former case the vital processes will merely be intensified in their course; in the latter and usual case the result will be determined according to the part of metabolism excited or depressed. When the effect of a stimulus is to excite D continuously in a high degree without correspondingly increasing A, the result is a dying off—an atrophy. In the contrary case, when A remains continuously greater than D, the result is growth, increase and

reproduction of the cell. Experience proves, however, that A and D stand in a certain relation of mutual dependence to each other, with the result that when D has been increased by a stimulus, for example, A correspondingly increases during the stimulation, and continues to do so after its cessation, till the loss in living substance produced by the stimulation of D is eventually made good, and metabolic equilibrium is restored. The muscle may be taken as an example of this self-regulation of metabolism common to all living substance (Hering's Selbststeuerung des Stoffwechsels). When a muscle has been fatigued by some stimulation causing an enormous increase of D, there is a corresponding spontaneous increase in A. After some time the muscle is observed to have recovered. It has once more become capable of performing work; its metabolism is again in equilibrium.

The vital phenomena of the cell may be derived mechanically from metabolism and the changes it undergoes under the influence of stimuli. Our ability to do this will increase more rapidly as we become better acquainted with the details of the metabolism of the cell itself. The foregoing outline must be regarded, of course, as embodying only a fragmentary hypothesis, which can serve as a guide for further research only so long as it does not clash with facts, and which must be amplified, specialized and developed with the widening of specific knowledge regarding the cell's metabolism. The relations already known are so exceedingly complex that only by slow degrees can we pursue the investigation of separate fragments of the entire metabolic series. The differentiation of nucleus and protoplasm in the living substance of the cell alone gives rise to an extraordinary complication in the metabolic process, for these two parts of the cell stand in the most complicated correlation with

one another as well as with the environing medium—a fact of which the experiments made by vivisection in various free-living cell-forms have furnished abundant evidence. The farther such knowledge advances, the more rounded, clear and free from hypotheses will become our conception of the cell's metabolism. But the cell is the elementary component part of all organisms, and from the life of individual cells is constructed the life of the separate tissues and various organs, and thus of the entire organism. Hence the cell is the only vital element which the organism possesses, and therefore the investigation of the vital processes in its separate cells leads

ultimately to a knowledge regarding the mechanism of life in the whole.

Vegetable physiology is dealt with in the article : Physiology. For detaills of different parts of the animal body, see
 * ; ; ; ; ; ;, , &c.
 * ; ; ; ; ; ;, , &c.
 * ; ; ; ; ; ;, , &c.

The principal modern English textbooks of animal physiology are those of Sir Michael Foster (1885), A. E. Schäfer (1898), Noel Paton (1908), Halliburton (1909), and Starling (1909). See, however, the bibliographical notes to the separate articles.
 * (M. V.)

 PIACENZA (Lat. Placentia), a town and episcopal see of Emilia, Italy, the capital of the province of Piacenza, 42½ m. S.E. of Milan and 91 m. N.W. of Bologna by rail. Pop. (1906), 39,786. It lies on the Lombard plain, 217 ft. above sea-level, near the right bank of the Po, which here is crossed by road and railway bridges, just below the confluence of the Trebia. It is still surrounded by walls with bastions and fosse in a circuit of 4 m. The cathedral was erected between 1122 and 1233, in the Lombard Romanesque style, under the direction of Santo da Sambuceto, on the site of a church of the 9th century which had been destroyed by earthquake. The west front has three doors with curious pillared porches. The campanile is a massive square brick tower 223 ft. high; the iron cage attached to one of its windows was put up in 1495 by Ludovico il Moro for the confinement of persons guilty of treason or sacrilege. The crypt is a large church supported by one hundred columns. The entire edifice has been restored since 1898, and the frescoes by Guercino and Caracci, which decorate parts of its roof, though good in themselves, are inappropriate to its severe style. Sant' Antonino, which was the cathedral church till 877, is supposed to have been founded by St Victor, the first bishop of Piacenza, in the 4th century, and restored in 903; it was rebuilt in 1104, and altered in 1857. It was within its walls that the deputies of the Lombard League swore to the conditions of peace ratified in 1183 at Constance. The Gothic brick vestibule (Il Paradiso) on the north side is one of the older parts of the building. San Francesco, a spacious Gothic edifice begun by the Franciscans in 1278, was erected on the site of the palace of Ubertino Landi, a leader of the Ghibelline party. S. Savino, a fine Romanesque building of 903 (well restored in 1903), contains a mosaic pavement of this period with curious representations, including one of a game of chess. S. Sisto, which dates from 1499, and takes the place of the church founded in 874 by Angilberga (consort of the emperor Louis II.), lost its chief attraction when Raphael's Sistine Madonna (now in Dresden) was sold by the monks in 1754 to Frederick Augustus III. Its place, however, is occupied by a copy by Avanzini, and there are also several good intarsias by Bartolomeo da Busseto. S. Sepolcro and S. Maria della Campagna are both good early Renaissance churches; the latter is rich in frescoes by Pordenone. S. Anna, dating from 1334, was the church of the barefooted Carmelites. Of the secular buildings the most interesting is the Palazzo Communale, begun in 1281, one of the finest buildings of its kind in Italy. The square in front is known as the Piazza dei Cavalli, from the two bronze equestrian statues of Ranuccio (1620) and his father Alexander, prince of Parma, governor of the Netherlands (1625). Both were designed by Francesco Mocchi. The Palazzo dei Tribunali and the Palazzo degli Scoti are fine early Renaissance brick buildings with terra-cotta decorations. The huge Farnese palace was begun after Vignola's designs by Margaret of Austria in 1558, but it was never completed, and since 1800 it has been used as barracks. Other buildings or institutions of note are the old and the new bishop's palace, the fine theatre designed by Lotario Tomba in 1803, the great hospital dating from 1471, the library presented to the commune in 1846 by the marquis Ferdinando Landi, and the Passerini library founded in 1685. The Museo Civico, formed in 1903, contains collections of antiquities (though many of the Roman antiquities of Piacenza have passed to the museum of Parma), some good Flemish tapestries and a few pictures. The castle erected by Antonio da Sangallo the younger has been demolished. Piacenza is the junction of the Milan and Bologna line with that from Voghera and Turin. From Codogno, 7 m.