84 



THE MUSCLE TISSUES 



Sometimes when pieces of the intestinal wall are fixed, indications 

 of the contraction wave have been preserved. Small band-like 

 swellings running across the muscle sheet appear at regular intervals, 

 these contraction swellings involve different portions of adjacent 

 cells which do not function as independent units. When the intes- 

 tine or other organ involved is greatly distended, the muscle sheets 

 appear much thinner than in the relaxed state. With relaxation 

 after such expansions the cells slide back into their former position 

 and form a thicker coat. Capillaries extend through the connec- 

 tive tissue network surrounding the muscle cells and follow their 



Fig. 45. — Photograph of cross-section and longitudinal section of smooth muscle 

 in wall of frog's stomach. 



disposition as do the sympathetic nerves controlling their involun- 

 tary action. In the repair of smooth muscle there are evidences of 

 some degree of mitotic activity on the part of fully formed cells, 

 but in cases of extensive injury lesions are closed by scars of con- 

 nective tissue. 



CARDIAC MUSCLE. 



Beneath the foregut of the embryo, an endothelial tube surrounded 

 by mesenchyme is the forerunner of the heart and the base of its 

 connecting vessels, to which this ty])e of muscle is limited. The 

 processes of the early mesenchyme cells appear to continue with the 

 adjacent cells to form a syncytium, and from these the cardiac 

 muscle is derived by contiiuied multi])licati()n, gradual elongation, 

 and differentiation of intracellular fibrils. The developing cells, or 

 myoblasts, elongate and granules are said to a])pear first in the 



