Contractile Elements

CONTRACTILE ELEMENTS, specially differentiated internal regions of protoplasm, the change in shape of which leads to movement in space, i.e., the movement of the whole organism or a part of it. In the most primitive form, contractile elements are described as myonemes in protozoa, e.g., many ciliates—rather thick threads usually located in the outer layer of the body (ectoplasm) directly beneath the pellicle. When the animal contracts (very vigorously in ciliates), one can observe the shortening of the myonemes along with their simultaneous thickening. The most perfect development of contractile elements for protozoa is reached in the so-called muscular stalk of Vorticella. Koltsov's studies show that the myoneme of Vorticella consists of a strong outer sheath (pellicle), an outer layer of protoplasm (tecoplasm), an inner homogeneous kinoplasm, and a layer of dense longitudinal fibrils at the boundary of both parts of the plasma. Koltsov's experiments with osmotic and macerational effects on the myoneme of Vorticellae lead him to the conception that the fibrils are purely supporting formations and that the active role in the contraction process belongs to the kinoplasm. Generalizing this concept, Koltsov and Roskin suggest that the contractile element in a number of cases studied by them (ciliates, cells of lower and partly higher invertebrates) is the liquid kinoplasm, while the dense elongated fibrils play the role of purely skeletal formations. This view, which has some data in its favor, nevertheless encounters weighty objections—a number of facts speak in favor of fibrils as the principal contractile element. A further development of the myoneme type is the so-called myoepithelial cells in hydra, ascaris, and other lower invertebrates, presented in some modification also in the glands of higher invertebrates and vertebrates, including man; in particular, the m. dilatator pupillae belongs to this type of formation. The general principle of these formations is an isolated region of protoplasm with thick fibrils running through it, while the rest of the cell is represented by ordinary granular protoplasm with a nucleus, which performs (e.g., in hydra) at times the function of glandular epithelium. A further complication of contractile elements is the muscle cells of a number of invertebrates (leeches, ctenophores, etc.), where the contractile elements are located in the outer layer of cells, forming a hollow cylinder inside of which is located the liquid protoplasm (sarcoplasm) with a nucleus; the contractile elements of the leech are represented by ribbon-shaped fibrils. Further development of the same principle, i.e., the increase in the number of contractile elements—myofibrils—at the expense of a decrease in indifferent sarcoplasm, is found in the muscle cells of mollusks, as well as in the so-called smooth (visceral) muscle cells of vertebrates. Here, the sarcoplasm is reduced to a small layer around the nucleus and near the fibrils, while the entire cell is filled with myofibrils—thin threads running parallel to the longitudinal axis of the cell and possessing uniaxial birefringence. Some authors distinguish in such cells thick boundary fibrils, to which they attribute a supporting significance, and bundles of thinner threads occupying a central position and being properly the contractile elements. The boundaries of myofibrils in smooth muscles have not been established with certainty; there is an opinion that they continue uninterruptedly from one cell into another, forming a kind of syncytium. The highest form of contractile elements is represented by the myofibrils of striated muscles, found in higher invertebrates (mollusks, crustaceans, insects) and in vertebrates. The length and density of the fibrils vary greatly even within a single animal; in any case, they do not reach the end of the sometimes very long fiber of striated musculature. The diameter of human myofibrils is about 0.2 µ or somewhat less. Characteristic of these formations is their structure. They consist of individual segments (inocomms) separated by a thin dense partition (Z discs, telophragms, intermediate discs); each segment consists of a region of birefringent dense substance (anisotropic substance, Q disc) and two regions of a lighter homogeneous singly refracting substance (isotropic substance, I disc)—see Muscles, histology. The sarcoplasm located between the myofibrils, besides its nutritional function, is also attributed by some as participating in the process of muscle contraction. Contractile elements of muscle cells possess the fundamental manifestations of life—metabolism and growth—due to which they must be regarded not as derivatives of protoplasm (metaplasmatic formations), but as true cell organelles. Contractile elements develop, as has been traced mainly for vertebrates, by division, growth, fusion, and subsequent differentiation of chondriosomes. Heidenhain's view that myofibrils contain elementary submicroscopic units which are the genuine contractile elements is not shared by the majority of authors. (Paragraph translated from the end of the text on contractile elements by Y. Zalkind).

SOXHLET APPARATUS was proposed by Soxhlet in 1886 (and with some later improvements in 1891) for the sterilization of milk intended for feeding small children and various milk dilutions used for feeding. The Soxhlet apparatus consists of a pot (water bath) into which bottles of a certain shape and capacity ("Soxhlet flasks") placed on a stand are inserted. The bottles, each of which is poured with infant food in an amount corresponding to a single feeding, are covered with tightly fitting rubber plates. The device is placed on fire and the water is heated to a boil. When the water is heated, steam is released from the liquid in the bottle, displacing the air from the space located above the liquid level and occupying this entire space. The boiling of water in the pot is maintained, according to Soxhlet's instructions, for 30–40 minutes. The temperature of the liquid in the bottles reaches 98–99.5°. Following this, the bottles are removed from the water bath and their contents are subjected to cooling, which is achieved either by cooling or with the help of artificial cooling. Upon cooling, the vapors contained in the bottles in the space between the liquid level and the rubber circle condense and a vacuum is formed in this space; thanks to this, the outside air rushes inside the bottle, but encountering an obstacle to penetration into them from the rubber plates, presses them and thus tightly closes the bottles. Thus, hermetic sealing is created, and consequently complete isolation of the contents of the bottle from the external

air with all the microorganisms contained in it (according to Soxhlet, in significant quantities). At the same time, thanks to the prolonged high heating of the nutrient liquid in the bottle, all the microbial population in it is killed, and under the influence of both these factors, the liquid turns out to be sterile and can maintain its sterility, according to Soxhlet, for a long time, up to 3–4 months. Thus, Soxhlet's task boils down to giving every mother a device that would give her the opportunity to sterilize the food intended for her child and maintain this sterility until the moment the child consumes the food. In imparting sterility to milk, in the complete removal of microbes from it, Soxhlet saw the resolution of the entire problem of artificial feeding. In this respect, Soxhlet was infinitely far from modern views. All modern understanding of the adequacy of food and its ability to be assimilated by the child, which is the main factor determining the successful development of the child during artificial feeding, was alien to Soxhlet, and the significance of microorganisms in the etiology of diarrhea was understood by him crudely and primitively in the form of direct harm to the child from the bacteria contained in his food. In the same way, Soxhlet was a stranger to the thought of the biological significance of species-specific milk. Proceeding from his views, Soxhlet did not stop at the demand that milk be boiled for 40 minutes, which from a modern point of view is a crude technique leading to complete denaturation of milk and to the destruction or at least a significant reduction in the vitamin content of the product. Despite the fact that some fundamental considerations laid down by Soxhlet at the base of his apparatus have been refuted by modern views, the apparatus itself, as a device intended for a specific purpose, has enormous significance, and both the device itself and the technological process of using it played a major role in the field dedicated to milk processing in general and in particular in the manufacture of baby food, in the milk kitchen. All devices currently used for "sterilization" of milk, regardless of their size and shape, repeat the Soxhlet apparatus. What was said by Soxhlet about fractional heating of each portion of baby food separately and about creating a hermetic seal until the moment the child consumes the baby food is currently leading in the technological routine of the milk kitchen. Literature: Soxhlet F., Ein verbessertes Verfahren der Milch-Sterilisierung, Münch. med. Wochenschrift, 1891, p. 335–353 (also sep. ed.—München, 1891). Ya. Zhorno.

Author signatures: Ya. Zhorno.