54 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



central nervous system, whether in or out of the body, continues 

 to exhibit the phenomenon of tone to a remarkable degree. In 

 most of the organs in which plain muscle occurs there are present 

 also numerous nerve cells, and it is therefore still a question as to 

 whether the tonic changes shown by this tissue depend upon a 

 property of the muscle itself or upon their intrinsic nerve cells. 

 Most observers adopt the former view. The importance of this 

 property of tone in the plain muscle tissues will be made fully 

 apparent in the descriptions of the physiology of the organs of cir- 

 culation and digestion. Plain muscle may exhibit also the phenome- 

 non of rhythmical activity, that is, under proper conditions it may 

 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, is so directly concerned with its functions as 

 an organ of circulation that it may be discussed 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. 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, 



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

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



