80 AN AMERICAN TEXT-BOOK OE PHYSIOLOGY. 



for it occurs in the absence of this sheath, both in the peripheral nerves and 

 in the central nervous system. The large class of non-medullated nerves 

 have the power of isolated conduction, and Donaldson reports that new-born 

 rats can make co-ordinated movements, although the nerves of both the 

 peripheral and central nervous systems do not acquire a medullary sheath 

 until several days after birth. It is not likely that the neuroglia cells are 

 essential to isolated conduction within the central nervous system, for this 

 occurs in its absence in the peripheral nerves. Although the neurilemma, by 

 separating the axis-cylinders of adjacent fibres, may make the insulation more 

 complete, it is probably not the real cause of isolated conduction. A break 

 of the protoplasmic continuity of the nerve protoplasm stops conduction, and 

 conduction fails wherever protoplasmic continuity is lacking. 



An apparent contradiction to the rule that absolute continuity of nervous 

 matter is essential to conduction by nerves, is to be found in the phenomenon 

 known as " Hering's Paradoxical Contraction." This will be explained 

 later (see p. 157, d). 



(b) Distribution of Excitation by Branches of Nerves. — Nerve-fibres some- 

 times branch in their passage along the peripheral nerves, but most of the 

 branches which are seen to be given off from the nerve-trunks are composed 

 of bundles of nerve-fibres which have merely separated off from the rest. 

 After the nerves have entered a peripheral organ, or the central nervous system, 

 the axis-cylinders may give off branches. Thus in muscles, and toastill greater 

 degree in the electric organs of certain fish, the nerve-fibre and its axis-cylinder 

 may divide again and again, or after entering the spinal cord the fibre may be 

 seen to give off a great many lateral branches — collaterals, as they are called. 

 It is not known whether in such cases the fibrillse of the axis-cylinder give 

 off branches, or whether they simply separate, a part of them entering the 

 branch while the rest of them continue on in the main fibre. Though the 

 exciting process does not pass from fibre to fibre, it probably involves in a 

 greater or less degree all the elements of the same fibre, and passes into all its 

 branches. It is evident that where it is necessary for the irritation to be 

 localized, branching could not occur; but where a more general distribution 

 is permissible, especially where several parts of an organ ought to act at the 

 same instant, conduction through a single fibre which branches freely near its 

 termination would be useful. 



(c) Conduction in Muscles. — Each fibre of the muscles which move the 

 bones — the skeletal muscles, as they are sometimes called — is physiologically 

 independent of the rest. The sarcolemma prevents not only continuity, but 

 contiguity of the muscle-substance of the separate fibres, and there is no cross 

 conduction from fibre to fibre. That this is so is proved by an experiment, such 

 as was described on page 15, in which unipolar excitation of the part of the 

 fibres of a curarized sartorius muscle results in a contraction strictly con- 

 fined to the fibres which are subjected to the irritating current. Each of the 

 separate muscle-fibres is supplied by at least one nerve-fibre or a branch 

 of a fibre, and, under normal conditions, only acts when stimulated by 



