78 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



part of a nerve- or muscle-fibre acts as a barrier to conduction. If a nerve be cut 

 through, the irritability and conductivity remain for a considerable time in the 

 severed extremities, but communication between them is lost, and remains absent 

 however well the cut extremities may be adjusted. The nerve-impulse is not 

 transmitted through the nerve-substance as electricity is transmitted along a 

 wire: join the cut ends of a wire, and the contact suffices for the passage of 

 the current ; join the cut ends of a nerve, and the nerve-impulse cannot pass. 

 Any severe injury to a nerve alters the protoplasmic structure and prevents 

 the chemical and physical processes through which conductivity is made 

 possible. It is probable that the same may be said of all forms of liv- 

 ing cells, and the absence of protoplasmic continuity would seem to be an 

 explanation of the fact that nerve- and muscle-fibres which lie close together 

 may physiologically act as separate mechanisms. 



Even in the case of apparently homogeneous protoplasm there is probably 

 a definite structural relation of the finest particles, and upon this the physi- 

 ological properties of the substance depends. Slight physical and chemical 

 alterations suffice to change the rate and strength of the conduction process, 

 and the power to conduct is altogether lost if the protoplasm is so altered that 

 it dies. 



The relation of conductivity to structure of cell-protoplasm is illustrated in 

 the effects of degeneration and regeneration upon the physiological properties of 

 the nerve-fibre (see p. 69). The life of the nerve-fibre is dependent on influ- 

 ences exerted upon it by the body of the cell of which it is a branch. When 

 any part of the fibre is injured it loses its power to conduct, and the portion 

 of the fibre separated by this block from the body of the cell soon dies. The 

 irritability and conductivity are wholly lost at the end of a period varying 

 from four davs to several weeks, the time differing in different kinds of nerves, 

 and the fibre begins to undergo degeneration. The axis-cylinder and the 

 myelin are seen to break up and are then absorbed, apparently with the 

 assistance of the nuclei which normally lie just inside the neurilemma, 

 and which at this time proliferate greatly and come to occupy most of 

 the lumen of the tube. The process of absorption is nearly complete at 

 the end of a fortnight after the injuy. Under suitable conditions, however, 

 regeneration may occur, and as this takes place there is a recovery of physi- 

 ological activities. The order in which conductivity and irritability return is 

 instructive. Howell and Huber 1 made a careful study of this subject. They 

 found that the many nuclei which develop during degeneration are apparently 

 the source of new protoplasm, which is seen to accumulate in the old sheath until 

 a continuous band of protoplasm is formed. About this thread of protoplasm 

 a new membranous sheath develops, and thus is built up what closely resembles 

 an embryonic nerve-fibre. The embryonic fibre formed in the peripheral end 

 of the regenerating nerve joins that of the central end in the cicatricial tissue 

 which has been deposited at the point of injury. Thus a temporary nerve- 

 fibre is formed and united to the undegenerated part of the old fibre, and this 

 1 Journal of Physiology, 1892, vol. xiii. p. 381. 



