DEVELOPMENT. 



303 



contracted, so that an indistinct appearance of striation may thus be produced. 

 The cells are united by a small amount of intercellular cementing substance which 

 becomes stained by nitrate of silver. In some parts intercellular spaces may occur, 

 bridged across as in epithelia by fine denticulations of the cells. 



They are generally collected into larger and smaller fasciculi, which in many 

 cases cross one another and interlace. The fasciculi are attached at their ends by 

 connective tissue to the membranous and firmer parts where they occur. In some 

 cases the attachment of the plain muscular cells takes place by means of elastic 

 fibres which bifurcate at the end of the muscular cell. The two branches extend 

 along either side of this and are firmly attached to it. In other cases again, accord- 

 ing to "Watney, the attachment may take place through the medium of connective 

 tissue corpuscles, the branches of which embrace the muscular cell in like manner. 



Distribution. The plain muscular tissue is for the most part disposed in the 

 coats of the membranous viscera. It is met with in the lower half of the gullet, the 



Fig. 348. MUSCULAR FIBRE-CELLS FROM THE UTERUS THREE WEEKS AFTER DELIVERY, 



TREATED WITH ACETIC ACID, MAGNIFIED 350 DIAMETERS. (Kolliker. ) 



a, nuclei ; 7, fat-granules. 



stomach, and the whole intestinal canal ; that is, both in the muscular 



coat of the alimentary canal, and also as a layer in the tissue of the 



mucous membrane, and in the villi ; in the trachea and bronchial 



tubes, in the bladder and ureters, and the ducts of the larger glands 



generally, in the uterus and its appendages, in the corpora cavernosa 



of both sexes, in the prostate gland, in the spleen, in the muscle of 



Miiller at the back of the orbit, and in the ciliary muscle and iris. 



The middle coat of the arteries, the coats of many veins and those of 



the larger lymphatics contain plain muscular tissue. In the skin it 



is present in the tubules of the sweat glands, in the form of minute 



muscles attached to the hair-follicles, and in the dartos or subcutaneous 



tissue of the scrotum. Numerous nerves, chiefly of the pale variety, 



are supplied to this tissue ; before their ultimate distribution they 



frequently come into connection with microscopic ganglia. The 



tissue receives blood-vessels, but these are far fewer in proportion than those of 



voluntary muscle. In some situations, as in the wall of the stomach and intestine, 



abundant lymphatic plexuses are found in close relation to the muscular layers. 



Development. The elements of the plain or unstriped muscular tissue are 

 derived from embryonic nucleated cells, consisting of the usual granular-looking 

 protoplasm. These cells become lengthened out, pointed at the ends, and flattened 

 with elongation of the nucleus, whilst their substance becomes longitudinally fibril- 

 lated and anisotropous, and acquires its permanent condition and characteristic 

 properties. 



The great increase in the muscular tissue of the uterus during gestation takes 

 place both by elongation and thickening of the pre-existing fibre-cells of which that 

 non-striated tissue consists, and it is said also by the development of new muscular 

 fibre-cells from small nucleated, granular cells lying in the tissue. In the shrinking 

 of the uterus after parturition the fibre-cells diminish to their previous size ; many 

 of them become filled with fat granules (fig. 348), and eventually many are doubtless 

 removed by absorption. 



Regeneration of plain muscle after artificially-produced lesions has been seen to 

 be accompanied by karyokinetic multiplication of the muscle-cells adjacent to the 

 lesion (in the newt by Stilling and Pfitzner). 



