352 DEVELOPMENT OF NERVES. 



termination is in the form of a ramification of the axis-cylinder on the surface of the 

 fibre within the sarcolemma. 1 



The termination of motor nerves in special granular expansions within the sarcolemma was 

 first noticed by Doyere in insect-muscles. The aborescent termination of the axis-cylinder was 

 discovered in the frog by Kiihne in 1862. In the same year the end-plates were recognised by 

 Rouget in the lizard, and in 1863 by W. Krause in mammals. The last named observer was 

 the first to describe the termination of the axis-cylinder as a ramified expansion imbedded in 

 granular substance, but maintained that the whole structure lay outside the sarcolemma. In 

 this opinion Krause is supported by Kolliker, but by far the majority of observers regard the 

 whole end-organ as hypolemmal in position. Engelmann and Fcettinger have been led from 

 observations upon insect-muscles to the conclusion that the expansion of the nerve-fibre comes 

 into actual continuity with the isotropous substance, i.e., with the sarcoplasm. of the muscular 

 fibre. But the effect of section of a motor nerve in the living animal the resulting degen- 

 eration extending no further into the muscular fibre than the end-plate itself is a strong 

 argument against the existence of any such anatomical continuity. 



DEVELOPMENT OF THE NERVES. ' 



The embryonic development of the nerves has already been treated of in the 

 part of this work devoted to Embryology. It was there shown that all nerve-fibres 

 and nerve-cells, whether belonging to the central nervous system or to the peripheral 

 and sympathetic nerves, are originally derived from the neural or muro-sensory 

 epiblast, and that in the case of the afferent nerve-fibres, such as those of the 

 posterior roots, the axis-cylinders grow from the cells of origin (neuroblasts of the 

 spinal ganglia), both centripetally into the nerve-centre, and centrifugally towards 

 the peripheral sensory parts, while in the case of the efferent nerve-fibres, those 

 namely of the anterior roots, the axis-cylinders grow only centrifugally from their 

 cells of origin, which here lie within the nerve-centre (neuroblasts of the nerve- 

 centres), and thence eventually pass to and unite with the muscular fibres. So far 

 as is known, this is the only mode of development of nerve-fibres, viz., as out-growths 

 from nerve-cells or neuroblasts, and they always, whether in the nerve-centres or in 

 the nerve-trunks, at first appear as pale fibres, destitute both of primitive and of 

 medullated sheath. 



It appears somewhat doubtful whether the pale fibres which are thus first formed 

 are single axis-cylinders or bundles of such. However this may be, they early 

 become surrounded by cells from the adjacent mesoblast, which penetrate also 

 between them, and eventually produce the connective tissue of the nerve-sheath 

 (epineurium, perineurium, endoneurium). In the nerve-centres very little connective 

 tissue passes between the nerve-fibres, which are there supported by the spongio- 

 blasts (see Embryology, p. 58). These are cells which have a common 

 (epiblastic) origin with the neuroblasts, although their function, according 

 to His, is early differentiated. 2 From the spongioblasts, neuroglia cells appear ulti- 

 mately to be produced, and these, within the central nervous system, take on much 

 the same supporting function which is elsewhere fulfilled mainly by connective tissue. 



The medullary sheath does not make its appearance until a comparatively late 

 period of embryonic life, and there is much doubt as to its mode of formation. It 

 first appears as a thin layer of myelin, not unfrequently interrupted, which closely 

 ensheaths the axis-cylinder, and is itself ensheathed by the nucleated neurolemma cr 

 sheath of Schwann which has been previously formed. Vignal refers the formation 



1 For an account of the variations which are met with in different animals, illustrated by a large 

 number of drawings, and for a discussion of many disputed points regarding the details of structure of 

 these organs, the reader is referred to a paper by Prof. W. Kuhne in the "Zeitschrift f. Biologie," Bd. 

 23, 1886. 



2 Ramon y Cajal, however, states that many of the neuroblasts are directly derived from cells which 

 are identical with the spongioblasts of His. 



