54 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



contract and relax rhythmically like heart tissue.* Such movements 

 have been observed and studied upon the plain muscle of the ureter, 

 the bladder, the esophagus, stomach, and other portions of the 

 alimentary canal, the spleen, the blood-vessels, etc. This property 

 seems to be very unequally distributed among the different kinds 

 of plain muscle found in the same or different animals, but this 

 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, are so directly concerned with its functions 

 as an organ of circulation that they may be discussed more profit- 

 ably 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. Epithelium with motile cilia is found lining 

 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 demon- 

 strated to be motile in the frog, but whether this is true for the mam- 

 mal has not been shown. So also in the neck of the urinif erous 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 



*Engelmann, "Archiv f. d. ges. Physiologie," 2, 243, 1869. Stiles, 

 "Amer. Jour, of Physiology," 5, 338, 1901. 



