538 Caroline McGill. 



In a section through the large intestine of an eight-month human 

 foetus the author observed rather distinct contraction nodes. At that 

 age the contraction waves are quite irregular in outline. 



Summary. 



A. Structure of resting smooth muscle. 



1. Smooth muscle in chick, pig and man arises as a syncytium 

 and the syncytial structure persists in most instances in the adult. 



2. Adult smooth muscle may show one of two and possibly of 

 three types of structure. In type 1 there is a very distinct syncytial 

 arrangement. The fibers are joined by both side and end anasto- 

 moses. In type 2 the muscle fibers show few side anastomoses, but 

 end to end union still persists. There is possibly a third type of 

 muscle with no anastomoses between the fibers, each forming an 

 individual cell. 



3. Muscle of type 1 was found in portions of the digestive and 

 urino-genital tracts of adult tectums, chick, pig, cat, dog and man 

 and of the arterial muscle of pig, ox and man. The tissue consists 

 of the much elongated muscle nuclei, each surrounded by a granular 

 protoplasmic reticulum and outside of this a layer of myofibrillae 

 embedded in clear protoplasm. Both granular protoplasm and myo- 

 fibrillar may be continuous from cell to cell. Where there is much 

 granular protoplasm there are few myofibrilla? and vice versa. 



4. In muscle of type 2 the loss of side anastomoses is probably 

 due to rapid elongation of the central part of the fiber during his- 

 togenesis, crowding the anastomoses toward the ends. This type was 

 found here and there in all muscle studied. The myofibrillae pass 

 continuously from cell to cell through the broad end anastomoses. 

 Where there is much granular protoplasm it may also help to form 

 the anastomoses. 



5. Muscle of type 3, though it is the type usually described, was 

 found hard to demonstrate. Frequently, especially in arterial mus- 

 cle, what appear to be isolated muscle cells are seen. They are long, 

 spindle-shaped elements, each with a central nucleus and surround- 

 ing this the sarcoplasm filled with myofibrillae. The cell body is 



