36 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



the fibre, may indirectly influence the whole, because the condition of activity 

 which it excites at the point of application is transmitted by the muscle-sub- 

 stance throughout the extent of the fibre. 



Irritability and conductivity are not confined to contractile mechan- 

 ism. They are possessed to a still higher degree by nervous tissues, which are 

 not found to have the power of movement. The nervous system is composed 

 of nerve-cells and nerve-fibres. The nerve-cells are located chiefly within the 

 brain and spinal cord, a smaller number being found in the spinal ganglia 

 and at special points along the course of certain nerve-fibres. The active part 

 of the nerve-fibre is the axis-cylinder, which is an outgrowth from a nerve-cell, 

 and which outside of the central nervous system acquires a delicate membran- 

 ous sheath, the neurilemma, which invests it as the sarcolemma does the muscle- 

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

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

 cylinder 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 muscles 

 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 

 does not prevent it from being classed with other irritable forms of living cell- 

 substance as protoplasm. In spite of differences in structure and composition, 

 nerve protoplasm and muscle protoplasm are found to have many points of 



