Physiology of the Muscles. 489 



frog, however, is covered with ciliated epithelium, the cilia of which 

 operate to work the food inwards towards the gullet. The evolution or 

 differentiation of such organs as these minute self-acting muscles, ante- 

 dates in animal history the differentiation of nerves and muscles proper. 

 The evolution of muscles is a process which must have begun in the 

 first animal whose body was big enough to admit of flexibility. 



Amongst the lowest animals in which there is a definite nervous sys- 

 tem, we find also well defined and correlated muscular action of parts. 

 In the Medusa, or Jelly-fish, which is perhaps the lowest in which a 

 clearly defined nervous system exists, the shape of the animal being like 

 an umbrella, the simple ganglia are distributed all around the margin 

 of the animal ; and when a stimulus, as a touch, blow or stroke is ap- 

 plied to any part of this margin, it is instantly conveyed to every other 

 part, and every muscle is thus stimulated to contraction at once, caus- 

 ing the closing of the umbrella, the expulsion of the water, and the back- 

 ward rebound of the animal. 



Vegetable protoplasm is contracted by electrical stimulus, and is dif- 

 ferentiated in some cases to contraction from nervous stimulation gen- 

 erated in the tissues of the plant. ( See chap. 54.) It has never been 

 settled precisely what the nature of the nervous current is. But it seems 

 to be generally supposed to be something akin to a galvanic current. 

 There are many ways in which the movement of ether makes itself ap- 

 parent ; and these different modes appear to depend upion the conditions 

 under which the movement takes place, just as the force of a head of 

 water may show itself in an unconfined rush over a dam, or may propel 

 an undershot wheel, an overshot wheel, a turbine or a hydraulic ram, 

 all exhibiting different forms of movement, arising from the same 

 source. The ether, like the water, when descending from a position of 

 potential energy, may do so through various mediums, and so exhibit 

 itself in various forms. Again, there are waves of different sizes pos- 

 sible in the same fluid, as water for example. The wave produced by 

 dropping in a shot is greatly different from that produced by the tum- 

 bling of an iceberg ; and the motions possible in a bucket of water but 

 little resemble those produced in the sea. So we find that the waves 

 set up in ether are of different sorts and lengths, depending upon the 

 nature of the body from which they originate. The waves excited by a 

 tallow candle differ from those of the arc light. So there is a great dif- 

 ference in ethereal currents. The electric spark, the direct galvanic 

 current, the galvanic induction current, the magnetic current, the ther- 

 mo-electric current, all differ in their origin, and more or less in their 

 manifestations, but are nevertheless of the same family. There is am- 

 ple reason to assign a place in the same list to nervous energy. The in- 

 timate nature of the difference between the various forms of ethereal en 



