78 HISTOLOGY. 



which lines the oesophagus, a part of which is shown in the figure, is seen 

 to be surrounded by mesenchymal tissue in which the smooth muscle cells 

 are being differentiated. There is a layer, c. m., in which the cells have 

 become spindle-shaped, and since they are parallel and close together, 

 they form a band encircling the oesophagus. Outside of this there is a 

 broader layer of elongated cells, 1. m., all running lengthwise of the oesoph- 

 agus and therefore cut across in this section. This layer of longitudinal 

 muscle passes into mesenchymal tissue on the outside. The figure illus- 

 trates that smooth muscle cells are elongated mesenchymal cells, gener- 

 ally parallel and ar- 

 ranged in layers. 

 In the embryonic 

 stage the are con- 



FIG. 89. SMOOTH MUSCLE FIBERS PROM THE SMALL INTESTINE OF , i i 



A FROG, x 240. nected by proto 



plasmic processes. 



Smooth muscle cells in the adult may occur singly or in the form of 

 interlacing networks. Generally they are in layers and so closely packed 

 that separate cells are hard to follow. Moreover they often extend be- 

 yond the planes of the section so that only portions of them are included 

 in the specimen examined. If a piece of fresh tissue is treated with a 

 ^5' , aqueous solution of potassium hydrate or 20% nitric acid, the cells 

 may be shaken apart, and appear as in Fig. 89. They vary in length from 

 0.02 mm. in some blood vessels to 0.5 mm. in the pregnant uterus; in the 

 intestine they are said to be about 0.2 mm. Their width ranges around 

 0.005 mm - (5,")- They are fusiform or cylindrical, 

 rarely being branched as has been recorded for muscle 

 cells in the bladder, the ductus deferens, and the aorta 

 (Fig. 156, p. 131). 



The nucleus, situated near the center of the cell, ^^SZLS-SSus 

 is cylindrical, with its chromatin in a network and in IG- SMOOTH ^MUSCLE 



i -,i i i -re 11 FIBERS FROM THE 



masses lining the nuclear membrane. In fa vorable prep- ARTERY OF A DOG 



arations it has been observed to contain several nucleoli, 

 and a diplosome has been found just outside of its longitudinal border. 

 When the muscle cell contracts the nucleus shortens and may be bent 

 or spirally twisted, Fig. 90. (Such nuclei have been interpreted as distor- 

 tions of resting nuclei caused by the contraction of neighboring cells.) 



The protoplasm of the smooth muscle cells early produces coarse 

 fibrils called border fibrils [myoglia], since they tend to be at the periphery 

 of the cell. They are said to extend from cell to cell, which is made pos- 

 sible by the syncytial arrangement of mesenchyma. In one interesting 

 but unique instance, the fibrils from the mesentery of a salamander showed 



