238 



ELEMENTS OF BIOLOGY 



Neuroid Transmission. The phenomenon o£ excitation-trans- 

 mission takes place in more than one type of tissue. The more 

 famiUar and the more striking effects are those involving the nerv- 

 ous mechanism, which is highly organized, and, so far as known, 



has no other function. Less familiar 

 are the correlative influences trans- 

 mitted by other tissues; they are less 

 prominent but none the less impor- 

 tant. Transmission over tissues other 

 than nerves has been called neuroid 

 TRANSMISSION. Scvcral examples will 

 serve to illustrate its role in unifving 

 the organism. 

 The lining of the respiratory pas- 

 sages of air-breathing vertebrates is composed of a layer of epithelial 

 cells, the exposed surfaces of which are ciliated. The cilia all beat 

 toward the nares, thus assuring an outward flow of the liquid that 



Fig. 163. — Diagram showing the 

 passage of coordinated waves o£ 

 ciliary beat along a ciliated epi- 

 thelial layer. 



P/GMT ATRIUM 



RIGHT VENTRICLE 



LEFT ATRIUM 



LEFT VENTRICLE 



Fig. 164. — Diagram to show the order of contraction in the beat of the human 

 heart. The region of the right atrium in which the beat is initiated is the homologue 

 of the sinus venosus of the hearts of lower vertebrates. 



moistens the passages. This ciliary beat is a coordinated effort but 

 the ciliated cells have no direct connections with the nervous system. 

 It is generally thought that the coordination of beat is effected by 

 the cell-to-cell contact throughout the epithelium (Fig. 163). 



