GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 21 



Irritability and conductivity are not confined to contractile mechan- 

 isms. They are possessed in a still higher degree by nerve-cells, neurones, 

 as they are called, which have not thus far been found to have the power of 

 movement, except that which is associated with the growth of a cell. Each 

 neurone is composed of a body and one or more branches. The bodies of the 

 nerve-cells are located chiefly in the spinal cord and brain, a smaller number 

 being found in the spinal ganglia and in the ganglia of the so-called sympa- 

 thetic system. The branches of a neurone are of two kinds, an axis-cylinder 

 process, or axone, which frequently carries at its extremity a specially formed 

 organ, through which it is able to excite to action the cells with which it 

 comes in contact, and protoplasmic processes, or dendrites, which have no 

 such exciting mechanism, and are destined to receive excitation and transmit 

 it to the body of the nerve-cell. Outside the central nervous system, at 

 least, the axone and the dendrite acquire a delicate membranous sheath, 

 the neurilemma, which invests it as the sarcolemma does the muscle-fibre. 

 The branches of nerve-cells together with their sheaths form the nerve- 

 fibres. There are two classes of nerve-fibres, medullated and non-medullated, 

 which are distinguished by the fact that the former has between the axis- 

 cyliuder and the neurilemma another covering composed of fatty material, 

 called the medullary sheath, while in the latter this is absent. Just as 

 it is the special function of the muscle-fibre to change its form when it 

 is excited, so it is the special function of the nerve-fibre to transmit the 

 condition of activity excited at one end throughout its length, and to 

 awaken to action the cell with which it communicates. Nerve-fibres are 

 the paths of communication between nerve-cells in the central nervous sys- 

 tem, between sense-organs at the surface of the body and the nerve-cells, 

 and between the nerve-cells and the muscle- and gland-cells. Nerve- 

 fibres are distinguished as afferent and efferent, or centripetal and centrifugal, 

 according as they carry impulses from the surface of the body inward or from 

 the central nervous system outward. Further, they receive names according 

 to the character of the activity which they excite : those which excite muscle- 

 fibres to contract are called motor nerves; those distributed to the museles 

 in the walls of blood-vessels, vaso-motor; those which stimulate gland-cells to 

 action, secretory; those which influence certain nerve-cells in the brain and so 

 cause sensations, sensory. Still other names are given, as "trophic" to fibres 

 which are supposed to have a nutritive function, and " inhibitory " to those 

 which check the activities of various organs. The method of conduction is the 

 same in all these cases, the result depending wholly on the organ stimulated. 



Nerve-fibres do not run for any distance separately, but always in company 

 with others. Thus large nerve-trunks may be formed, as in the case of the 

 nerves to the limbs, in which afferent and efferent fibres run side by side, the 

 whole being bound together into a compact bundle by connective tissue. The 

 separate fibres, though thus grouped together, are anatomically and physiologi- 

 cally as distinct as the wires of an ocean cable; that these many strands are 

 bound together is of anatomical interest, but has little physiological significance. 



The active substance of the nerve-fibre does not show contractility, but this 



