96 INVERTEBRATE PHYSIOLOGY 



and "fast" muscle fibers having different properties and separate nerve 

 supplies. The "slow" system works by virtue of the distributed end-plate 

 innervation which ensures a uniform depolarization of the muscle fiber 

 and an extensive, though graded, contraction in response to this. It is this 

 same distributed end-plate supply which enables insect muscle to function 

 in the extraordinary mineral environments which insect haemolymph often 

 presents, media in which the mechanism for establishing the vertebrate 

 propagated action potential would be immediately paralyzed. 



Up to the present time no rational explanation has been offered as to 

 why there are these "slow"- and "fast"-fiber mechanisms in the large 

 Crustacea. The answer may lie in a consideration of the insects ; for, as has 

 been pointed out, the highly economical "slow"- and "fast"-fiber system is 

 admirably suited to the special problem of controlling a very small muscle 

 composed of few fibers. If we suppose that the ancestral arthropods from 

 which the Crustacea and insects were derived were very small animals and 

 that the "slow"/"fast"-fiber mechanisms were evolved to meet their needs, 

 then it is readily possible to explain their persistence in the larger deca- 

 pods. On the grounds of performance there is little to choose between 

 vertebrate and arthropod. 



Summary 



(1) The mechanism of production of smoothly controlled movement 

 by the muscles of insects is discussed in relation to their innervation. The 

 small number of muscle fibers available makes it necessary to seek some 

 different phenomena from those familiar in vertebrates. 



(2) Each muscle in an insect is usually supplied by two axons and 

 itself constitutes a complete, single motor unit. Some muscles are how- 

 ever composed of a few units attached to a common apodeme, each unit of 

 which is served by separate axons. 



(3) The axons usually supply several end plates to each muscle fiber, 

 evenly distributed along its length. In many cases individual end plates re- 

 ceive branches from each of the two axons. 



(4) One of the two axons, the "fast" axon (F), evokes a large, twitch- 

 like response following each impulse. The other, the "slow" axon (S), pro- 

 duces a mechanical response only when impulses in excess of about 15 per 

 second pass down it. "Slow"-fiber responses always consist of smooth and 

 relatively slow contractions ; there is a very large increase in the rate of 

 contraction of the muscle with increasing frequency of stimulation of the 

 nerve and also in the final tension reached. 



(5) The "fast" fiber innervates every muscle fiber and produces large 

 electrical responses. These are formed of large end-plate potentials and 



