V. MOVEMENTS OF THE ALIMENTARY CANAL, 

 BLADDER, AND URETER. 



PLAIN MUSCLE-TISSUE. 



THE movements of the alimentary canal and the organs concerned in mic- 

 turition are effected for the most part through the agency of- plain muscle- 

 tissue. The general properties of this tissue have been referred to in the 

 section upon the Physiology of Muscle and Nerve, but it seems appropriate in 

 this connection to again call attention to some points in its general physiology 

 and histology, inasmuch as the character of the movements to be described 

 depends so much upon the fundamental properties exhibited by this variety of 

 muscle-tissue. Plain muscle as it is found in the walls of the abdominal and 

 pelvic viscera is composed of masses of minute spindle-shaped cells whose size 

 is said to vary from 22 to 560 /j. in length and from 4 to 22 fj. in width, the 

 average size, according to Kolliker, being 100 to 200 p. in length and 4 to 6 // 

 in width. Each cell has an elongated nucleus, and its cytoplasm shows a 

 longitudinal fibrillation. Cross striation, such as occurs in cardiac and striped 

 muscle, is absent. These cells are united into more or less distinct bundles or 

 fibres, which run in a definite direction corresponding to the long axes of the 

 cells. The bundles of cells are united to form flat sheets of muscle of varying 

 thicknesses, which constitute part of the walls of the viscera and are distin- 

 guished usually as longitudinal and circular muscle-coats according as the cells 

 and bundles of cells have a direction with or at right angles to the long axis 

 of the viscus. The constituent cells are united to one another by cement- 

 substance, and according to several observers 1 there is a direct protoplasmic 

 continuity between neighboring cells an anatomical fact of interest, since it 

 makes possible the conduction of a wave of contraction directly from one cell 

 to another. Plain muscle-tissue, in some organs at least, e. g. the stomach, 

 intestines, bladder, and arteries, is under the control of motor nerves. There 

 must be, therefore, some connection between the nerve-fibres and the muscle- 

 tissue. The nature of this connection is not definitely established ; according 

 to Miller 2 the nerve-fibres terminate eventually in fine nerve-fibrils which run 

 in the cement-substance between the cells and send off small branches which 

 end in a swelling applied directly to the muscle-cell. Berkley 3 finds a similar 



1 See Boheman : Anatomischer Anzeiger, 1894, Bd. 10, No. 10. 



2 Archivfur mikroskopische Anatomic, 1892, Bd. 40. 



3 Anatomischer Anzeiger, 1893, Bd. 8. 



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