CHAPTER IV 

 CONTRACTILITY 



Introduction 



Contractility is one of the fundamental attributes 

 of protoplasm. It is the means whereby the organism 

 changes its size and shape, and in the animal world its 

 position in space. It is seen in its simplqgt form in the 

 Amoeba, where by retraction here and protrusion there of 

 the undifferentiated protoplasm surrounding the nucleus, 

 the animal is enabled to ingest foreign particles and move 

 from one place to the other. This simple mode of locomo- 

 tion is known as amoeboid movement. Even in the highest 

 organism this method is retained. It is found for instance 

 in the leucocytes of the blood and in the pigment layers 

 of the retina. 



Ascending in the animal scale we find certain cells 

 speciahsed to effect, through changes in their shape, move- 

 ments of certain organs or of the whole organism, such 

 movements showing the widest variation in their strength 

 and rate. This capacity for change in shape is associated 

 with the presence of fibrils which are laid down in the cell 

 substance. The fibrils are known as sarcosfyles, and the 

 protoplasm in which they lie, sarcoplasm. The cells in 

 which the power of contraction is most strongly developed 

 are characterised by a great complexity of the sarcostyles. 



Broadly speaking, two types of muscle cell are found, 

 the unstriated and the striated, these terms being referable 

 to the absence or presence of transverse-striation in the 



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