94 



MUSCULAR TISSUE 



Smooth muscular tissue occurs chiefly in the walls of the hollow or 

 tubular viscera. Its distribution may be classified as follows : 



(1) In the alimentary tract: lower portion of the esophagus, stom- 

 ach, small and large intestines. 



(2) In the respiratory system: trachea and bronchial tubes. 



(3) In the g enito-urinary system: ureter, bladder, urethra, penis, 

 prostate, vagina, uterus, oviduct, and ovary. 



(4) In the vascular system: ar- 

 teries, veins, and the larger lymphatic 

 vessels. 



(5) In the ducts of all secreting 

 glands: gall ducts and gall bladder, 

 and the ducts of the pancreas, sal- 

 ivary glands, testicle, etc. 



(6) It is also found in the cap- 

 sules of the spleen and lymphatic 

 nodes, in the skin, and in the intrin- 

 sic muscles of the eye. 



Small numbers of branching cells 

 have been described in the walls of 

 the urinary bladder and the large 

 arteries. 



Cardiac Muscle. Heart muscle 

 likewise takes origin from splanchnic 

 mesothelium, which soon becomes 

 modified into a loose-meshed syncy- 

 tium, in which all trace of the orig- 

 inal cellular element is lost. The 

 myoblast areas of stellate and irreg- 

 ular form contain a central oval nu- 

 cleus and a finely granular cytoplasm. 

 In a manner similar to that described 

 for smooth muscle histogenesis, the 

 myochondria form myofibrils which 

 extend for great lengths throughout 

 the sarcoplasmic meshwork. In car- 

 diac muscle the myofibrilla? subse- 

 quently acquire a cross striation. Adult muscle consists of stouter mus- 

 cle fibers or trabecula3 joined into an intricate close meshwork, by means 

 of less robust branches. The nuclei retain their axial position in the 



FIG. 101. Two STAGES IN THE HIS- 

 TOGENESIS OF SMOOTH MUSCLE, 

 FROM THE WALL OF THE ESOPHAGUS 

 OF A PIG EMBRYO. 

 A, 10 mm. stage of development. 

 The central nucleus of the mesenchy- 

 mal syncytium has become enlarged 

 and is enveloped by a greater mass 

 of cytoplasm. It represents a myo- 

 blast; the peripheral myochondria have 

 become aligned preparatory to fusion 

 to form a muscle fibril. B, 21 mm. 

 stage of development! Four adjacent 

 myoblasts, with peripheral stouter 

 myoglia fibrils and central more deli- 

 cate myofibrils. X 1500. 



