134 THE ELEMENTARY NERVOUS SYSTEM 



nounced polarity, for contraction normally begins in the 

 region of the sinus and progresses thence in sequence over 

 the auricle, the ventricle and the bulb. The fact that 

 by appropriate stimulation this sequence may be re- 

 versed gives the heart a more 

 diffuse character than the 

 small intestine shows. But 

 the uncertainty as to the 

 myogenic or neurogenic na- 

 ture of its beat leaves the 

 heart a less clear example of 

 these peculiarities than some 

 other instances. 



As diffuse transmission 

 is characteristic of the 

 nerve-net and polarized 

 transmission is a feature of 

 the synaptic system, it fol- 

 lows that the first signs of 

 polarization in the nerve- 

 net may be regarded as the 

 initial step in the process of 

 converting this organ into 

 one of a synaptic type. As 

 this process is apparently 

 accomplished by the elongation of the neurofibril constitu- 

 ents of the net in a particular direction, this feature may 

 be regarded as the anatomical indication of the coming 

 change. With the growth of such a feature the nerve-net 

 begins to lose its diffuse condition and its transmitting 

 fibrils come to form bands or trunks with some resem- 

 blance to nerves ; in other words, the nerve-net loses some 

 of its net-like character and comes to develop a partial 



Fio. 40. Diagram to illustrate the 

 preparation of the intestine in Mall's ex- 

 periment on intestinal polarization. A, 

 piece of intestine suspended by its mesen- 

 tery and freed by two transections, but 

 till in normal position. B, the same, but 

 with the piece of intestine healed into 

 place in reversed position, a, anterior; p, 

 poiterior. 



