1112 



GENERAL ANATOMY OR HISTOLOGY. 



and trabeculse of the spleen ; in the mucous membranes, forming the muscularis 

 mucosse ; in the skin, forming the arrectores pilorum, and also in the sweat- 

 glands ; in the arteries, veins, and lymphatics : in the iris and the ciliary muscle. 

 Plain or unstriped muscle is made up of spindle-shaped cells, called con- 

 tractile fibre-cells, collected into bundles and held together by a cement-substance 

 (Fig. 643). These bundles are further aggregated into larger bundles or 

 flattened bands, and bound together by ordinary connective tissue. 



The contractile fibre-cells are elongated, spindle-shaped, nucleated cells of 

 various lengths, averaging from ^-g- to -g-^ of an inch in length, and -j^Vcr to 

 3Ttnr f an i ncn i n breadth. On transverse section they are more or less poly- 

 hedral in shape, from mutual pressure. They present a faintly longitudinal 

 striated appearance, and consist of an elastic cell-wall containing a central bundle 

 of fibrillae, representing the contractile substance, and an oval or rod-like nu- 

 cleus, which includes, within a membrane, a fine network communicating at the 

 poles of the nucleus with the contractile fibres (Klein). The adhesive interstitial 

 cement-substance, which connects the fibre-cells together, represents the endomy- 

 sium, or delicate connective tissue which binds the fibres of striped muscular 

 tissue into fasciculi ; while the tissue connecting the individual bundles together 

 represents the perimysium. The unstriped muscle, as a rule, is not under the 

 control of the will, nor is the contraction rapid and involving the whole muscle, 

 as is the case with the voluntary muscles. The membranes which are composed 

 of the unstriped muscle slowly contract in a part of their extent, generally under 

 the influence of a mechanical stimulus, as that of distention or of cold; and then 

 the contracted part slowly relaxes while another portion of the membrane takes 

 up the contraction. This peculiarity of action is most strongly marked in the 

 intestines, constituting their vermicular motion. 



Cardiac Muscular Tissue. The fibres of the heart differ very remarkably from 

 those of other striped muscles. They are smaller by one-third, and their trans- 

 verse striae are by no means so distinct. The fibres 

 are made up of distinct quadrangular cells joined 

 end to end (Fig. 644). Each cell contains a clear 

 oval nucleus, situated near the centre of the cell. 

 The extremities of the cells have a tendency to 

 branch or divide, the subdivisions uniting with 

 offsets from other cells, and thus producing an an- 

 astomosis of the fibres. The connective tissue 

 between the bundles of fibres is much less than in 

 ordinary striped muscle, and no sarcolemma has 

 been proved to exist. 



Development of Muscle-Fibres. Voluntary mus- 

 cular fibres are developed from the mesoblast, the 

 embryonic cells of which elongate, show multiplica- 

 tion of nuclei, and eventually become striated; the 

 striation is first obvious at the side of the fibre, 

 spreads around the circumference, and ultimately 

 extends to the centre. The nuclei, at first situated 

 centrally, gradually pass out to assume their final 

 position immediately beneath the sarcolemma. In 

 the case of plain muscle the mesoblastic cells assume 

 a pointed shape at the extremities and become flattened, the nucleus also length- 

 ening out to its permanent rod-like form. 



Chemical Composition of Muscle. In chemical composition the muscular 

 fibres may be said, in round numbers, to consist of 75 per cent, of water, about 

 20 per cent, of proteids, 2 per cent, of fat, 1 per cent, of nitrogenous extractives 

 and carbohydrates, and 2 per cent, of salts, which are mainly potassium phosphate 

 and carbonate. 



1 



FIG. 644. Anastomosing 1 muscu- 

 lar fibres of the heart, seen in a longi- 

 tudinal section. On the right the 

 limits of the separate cells with 

 their nuclei are exhibited somewhat 

 diagrammatically. 



