HISTOGENESIS AND STRUCTURE 93 



proper, which are limited to the cells proper and are considerably more 

 delicate. It is believed that border fibrils may subsequently arise by 

 fusion of the more delicate fibrils. 



The function of the border fibrils is disputed, some claiming that 

 they serve to straighten the cell following contrac- 

 tion produced by the central fibrils, others claim- 

 ing that they have a contractile role similar to the 

 central myofibrils. Whatever their complete func- ':* . ^ 

 tion may be, they certainly seem to bind, together 9 

 with the connective tissue, the individual cells into ^ 

 a compact tissue in which coordinated movement, 



as in peristalsis of the intestine, becomes possi- r 



FIG. 100. SMOOTH 

 ble an obviously important condition. MUSCLE CELLS 



The oval or rod-shaped nucleus retains its cen- FROM THE WALL OF 

 tral location, and is surrounded by a mass of gran- THE HUMAN INTES- 

 ular, relatively undifferentiated sarcoplasm, con- Transection Hema- 

 taining mitochondria, lipoid, and glycogen gran- te in and eosin. X 750. 

 ules. It changes its shape with the phase and 



degree of contraction, occasionally even assuming a short, spiral form. 

 It has been shown (McGill, Amer. Jour. Anat., 9, 4, 1909) that during 

 contraction the nucleus decreases markedly in length and increases in 

 thickness ; and that the uniformly distributed chromatin granules stream 

 toward the poles, where they collect in coarse strands. This structural 

 intranuclear change is apparently unaccompanied by any change in 

 volume. 



Smooth muscle cells vary greatly in size from the shortest of about 

 50 microns, to some of 500 microns in length in the pregnant uterus. 

 When in the contracted condition, they show a number of broad, more 

 deeply staining contraction bands, very conspicuous in the smooth mus- 

 cle of the lower portion of the esophagus. As seen in transverse section 

 these fibers vary in size from a mere point up to their maximum diame- 

 ter, according as the section happens to pass through the end or through 

 the middle of a fiber. Because of its central location, the nucleus 

 is only found in the larger transections. 



Smooth muscle fibers may be joined together in interlacing groups 

 as in the wall of the uterus or bladder; or they may form broad mem- 

 branous layers as in the wall of the alimentary tract; or again, 

 they may form small isolated bundles, as in the skin. In any case, 

 the muscle bundles are united by a delicate network of connective 

 tissue. 



