THE PHENOMENON OF CONTRACTION. 57 



fact serves only to illustrate the point already sufficiently empha- 

 sized, that grouping one kind of tissue — e. g., plain muscle — into 

 a common class does not signify that the properties of all the mem- 

 bers of the group are identical. The question as to how far the phe- 

 nomenon of rhythmical contraction is entirely muscular and how far 

 it depends upon intrinsic nerve cells is a complex one; the answer 

 will probably vary for different organs, and the subject will therefore 

 be considered in the organs as they are treated. 



Cardiac Muscular Tissue. — As the muscle cells of cardiac 

 tissue are somewhat intermediate in structure between the striated 

 fibers of voluntary muscle and the cells of plain muscles, so their 

 physiological properties to some extent stand between these two 

 extremes. The rate of contraction, for instance, while slower than 

 that of the fibers of skeletal muscles, is more rapid than that of 

 plain muscle. The most striking peculiarity of heart muscle is, 

 however, its power of rhythmical contractility, and this, as well as 

 its other properties, is so directly concerned with its functions as 

 an organ of circulation that ib may be chscussed more profitably 

 in that connection. 



Ciliated Cells. — In the mammalian body the phenomenon of 

 contractility is exhibited not only by the well-defined muscular 

 tissue, but also by the leucocytes and especially by the cilia of the 

 ciliated epithelium. Epithelial cells with motile cilia are found lin- 

 ing the mucous membrane of the air-passages in the trachea, larynx, 

 bronchi, and nose, in the lacrimal duct and sac, in the genital pas- 

 sages, uterus and Fallopian tubes and the tubules of the epididymis, 

 and in the Eustachian tube and part of the middle ear. Similar 

 cells are found lining the ventricles of the brain and the central 

 canal of the cord. The cilia in this latter position have been 

 demonstrated to be motile in the frog, and according to an old 

 observation by Purkinje* the same is true for the mammalian 

 (sheep) embryo. So also in the neck of the uriniferous tubule 

 ciliated cells are said to occur, but whether they are motile or not has 

 not been demonstrated. In the internal ear and the olfactory mucous 

 membrane the so-called sense cells are also ciliated, but here at least 

 the cilia are probably not motile. Ordinarily each ciliated epithelial 

 cell carries a bunch of cilia, all of which contract together, but 

 motile protoplasmic prolongations of the cell may occur singly, as 

 is illustrated in the spermatozoa, for instance, and in many of the 

 protozoa and plant cells. In the lower forms of life cilia play 

 obviously a very important role in locomotion, the capture of food, 

 and respiration, and their form and manner of movement vary 

 greatly. The form of movement or manner of contraction was 



* Purkinje, " Muller's Archiy," 1836. 



