84 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



cle-fibres of the legs, which, when stimulated, contract and withdraw the foot 

 from the offending irritant. The sensory and motor nerves concerned in this 

 reflex act run for a considerable part of their course in the same nerve-trunk, 

 but the sensory impulses have no direct effect on the motor nerve-fibres, and 

 the roundabout course which has been described is the only way by which 

 they can influence them. 



It is probable that isolated conduction by separate fibres and their branches 

 holds good within the central nervous system, as elsewhere, otherwise we could 

 scarcely explain the localization of sensations, or co-ordinated movements. It 

 is possible that within the central nervous system the neuroglia may act to 

 secure isolated conduction. This question will be considered elsewhere. 



(6) Distribution of Excitation by Branches of Nerves. Nerve-fibres rarely 

 branch in their passage along the peripheral nerves. The branches which are 

 seen to be given off from the nerve-trunks are composed of bundles of nerve- 

 fibres which have separated off from the rest, but which remain intact. After 

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

 axis-cylinders may give off branches. Thus in muscles, and to a still 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 fibrillae 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 eyident 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. Each of the separate muscle-fibres is supplied 

 by at least one nerve-fibre, and, under normal conditions, only acts when 

 stimulated by the nerve. In the case of plant-cells, and of certain forms of 

 muscle-cells, about which there is a more or less definite wall or sheath, there 

 are little bridges of protoplasm binding the cells together. For example, 

 Engelmann describes the muscle of the intestines of the fly as composed of 

 striated cells, sheathed by sarcolemma, except where bound together by little 

 branches of sarcoplasma, which may act as conducting wires between the cells. 



There are certain cells, however, which have been supposed to be exceptions 

 to the rule that protoplasmic continuity is essential to conduction. The stri- 

 ated muscle-fibres of the heart are quite different from those of ordinary 



