Page:The New International Encyclopædia 1st ed. v. 14.djvu/191

MUSCULAR SYSTEM. 155 MUSCULAR SYSTEM. MUSCULAR SYSTEM, Evolition of. The muscular systuin (.■iinipiises those tissues or niasses uf tissue which have the property of con- tractility and whose function it is to accomplish the movements of the parts or of the whole of the animal body. Contractility is one of the prop- erties of simple protoplasm. Thus, the amceha progresses by the extension of some portion and the contraction of other parts of its body. The cortical layer of Paranitecium is a specially con- tractile region in the protoplasm. In the Stenlor and the stalk of Vorticella there are protoplasmic dill'crentiations that resemble muscle Kbres. Many of the outer-lying ectoderm cells of Hydra show considerable dilierentiation ; they are conical in shape with the broad surface outward. On this external surface a thin, cuticle-like layer has been demonstrated. Internally the cell ends in a contractile basal portion or process which lies parallel to the long axis of the body of the hydra and between the ectoderm and entoderm. The whole of these cells are contractile, but the long processes are specially so. These processes ■were formerly known as "neuro-museular,' but with the discovery of special nerve cells, the mus- cular function alone must be conceded to them. Like the muscle cells of vertebrates they con- tract when properly stimulated. Here then are ectodermic muscle cells, the entire protoplasm of which, like that of the amoeba, is irrital)le and contractile, but with a portion of the cell more clearly set apart to perform the contractile func- tion than in an_y protozoan. In the jelly-fishes, such as Aurelia. a muscle cell is likewise com- posed of two part.s: a contractile portion (which shows cross striations), and, attached to the striated portion, a protoplasmic, non-differen- tiated, non-contractile portion which may bear cilia on its external surface. This muscle-bear- ing epithelium is arranged in a bundle or ring around the edge of the sub-umbrella. It is the contraction of this muscular zone that propels the jellyfish through the water. In the tentacles and about the lips unstriated muscle-fibres occur. The contraction and extension of the body of the sea-anemone, as well as of its gullet and mesen- teries, and its ability to move slowly from the point of attachment, are accomplished by means of a well-developed muscular system. This con- sists of bands of longitudinal muscle fibres which run on the mesenteries from the base to the disk; of parietal muscles which pass obliquely across the lower and outer angle of the mesentery; and of a thin sheet of transverse muscles. The con- traction of the longitudinal muscles draws the animal toward the base, and that of the trans- yerse muscles causes the contracted animal to extend again. A band of circular muscles at the junction of column and disk causes the disk and contracted tentacles to be inclosed withiii the body of the anemone. In addition to the bands of nuiscles. scattered fil)res occur both in the body wall and in the gullet, which consist partly of spindle-shaped, nucleated fibres, and partly of such striated muscle processes as occur in Hydra. The latter are mostly found in the transverse muscles of the body and the tentacles, and are of entodermal origin, as also ai"e the muscle bands of the mesenteries. Other longi- tudinal muscles are of cctodermie origin. Some of the nuiscles. however, sink so far down into the middle layer or niesoglea as to seem to belong to it alone. This is significant because all the Vol, XIV.— 11 musculature of animals above the ccelenterates lies between the ectoderm and entoderm. The musculature of llatworms falls under two groups: (1) the dermal musculature, and (2) the dorso-ventral musculature. The dermal mus- culature lies either under the basal membrane of the epidermis or under the cuticle. It is com- posed of distinct layers. In each layer all the fil)res run in one and the same direction. There are longitudinal muscles, transverse and diago- luil nmscles. The musculature is stronger on the ventral creeping surface. In cestodes the diago- nal layer is replaced by a second eir<'ular layer. The dorso-ventral musculature runs from the dorsal to the ventral surface, and is much dis- placed at sexual maturity by the male and fe- male germ-glands. Below the cuticular layer of roundworms there is a well-developed muscu- lar layer in the form of a tube, and composed of outer circular fibres and inner longitudinal ones. This muscular tube effects the writhings and un- dulations of the body. As in the coelenterates, the muscle element is composed of a single cell, made up of a protoplasmic (often glandular) portion and a fibrillar part. The (ibrilhc rarely show any evidence of cross striation, the lateral lines are free from muscle fibres, and the longi- tudinal sheet is thus broken up into bands. The muscular system of mollusks is also well developed. The muscle fibres, both of bivalve.? and gastropods, are all of the unstriated sort, because of the slow movements of the animals. These fibres occur usually in distinct bands or sheets, and frequently are combined for greater power to form very large muscles, such as the two abductors of Anodonta, which close the shell. Other important muscles are those of the foot, including two protractors and one retractor; and in the snail those that work the radula of the mouth, the retractors of the horns and penis, and those that pull the head into the shell. The muscles are fastened to the shell, upon which they make certain distinct markings at the points of attachment, called 'scars.' The musculature of segmented worms, like that of the roundworms, is composed of a dermo- musculature tube which is composed of an outer circular and an inner longitudinal layer of fibres. The fibres are in the form of bands. The pharjTix, mouth, gut. para])odia, and septa> have special muscle fibres. In leeches, in addition to the circular and longitudinal bands of muscle fibres, there are bands reaching diagonally from the dorsal to the ventral surface of the body. Among arthropods the musculature of the Crus- tacea is well developed and very complex, but there is little evidence remaining of the dermo-nuiscular tube of worms. It is assumed that the dorsal and ventral pair of longitudinal nuiscles corre- spond to the four similarly situated bands of muscle in the Polycha'ta. Perhaps traces of the circular musculature of annelids are found in the muscles that are attached to the basal ])arts of the appendages on the one hand, and to the integument of the body on the other. Certainly the firmer exoskeleton makes greater localization of the nuisculature possible. In the limb-bearing portion of the abdomen and the thora.v there are paired dorsal and ventral muscle bands. In the limbless part of the abdomen there is a thick layer or tulie of longitudinal muscle fibres, inter- rupted at each septum by the connective tissue tliat separates the myomeres. Tlius the muscle