ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 197 



The study of nuclei which have been stretched, leads us to conclude that 

 there is a viscous substance in the constituent parts of the nucleus, and 

 notably in the nuclear filaments and the intermediate substance; the 

 nucleoli are more consistent, and offer greater resistance to the causes of 

 deformation. It is probable that the filaments of an intact nucleus 

 are coiled and not arranged in a reticulum, In parts that have been 

 stretched, filaments disposed in a parallel or radial manner are varicose, 

 and their ajipearance often recalls primitive nerve-fibrils. There is often 

 a partial fusion between the filaments and the intermediate substance ; 

 in certain cases the coalescence may be complete, and the appearance of 

 parts, or even of the whole of the nucleus, may be homogeneous. Putting 

 aside the nuclear membrane and the nucleoli, we may distinguish, in 

 deformed nuclei, filaments and a homogeneous intermediate substance ; 

 the former appear to be formed of nuclein or chromatin and an achro- 

 matic substance; nothing but the latter gives any evidence of the 

 existence of a reticulum of plastin. 



Structure of Nerve-fibre.* — Dr. P. Schiefferdecker has rein- 

 vestigated the often attacked problem of the minute structure of nervous 

 tissue. His main conclusions are as follows : — (1) The distinction of 

 medullary and non-medullary nerves is a real one. (2) When there is a 

 medullary sheath, both on peripheral and central fibres, this is doubly 

 interrupted, by Lantermann's indentations which separate the various seg- 

 ments, and by the more widely separated constrictions of Kanvier. Buth 

 affect the whole thickness of the medullary sheath, and are seen on living 

 fibres. (3) At both interruptions an intermediate substance lies between 

 the medullary portions. This is readily dissolved away, but with silver 

 solution and the like may be hardened. These annular plates (" Zwischen- 

 scheiben") are seen at Ranvier's constrictions, and funnels (" Zwischen- 

 trichter ") at Lantermann's indentations. (4) Substances find their way 

 into the axial cylinder most rapidly at the intermediate discs. They 

 must be of especial nutritive importance. 



(5) The medullary sheath possesses no peculiar nuclei. (6) While 

 all central fibres lie naked in the supporting substance, all the peripherals 

 have a connective tissue sheath, the sheath of Schwann. This begins at 

 the origin of the roots from the central organ. (7) In the non-medullary 

 this sheath lies close to the axial cylinder, in the medullary close to the 

 medullary sheath, so close indeed to the latter that its contour is not 

 usually visible. (8) Characteristic nuclei, surrounded by more or less 

 protoplasm occur at intervals, and project markedly on the inner surface 

 of the fibre. (9) The sheath forms a tube corresponding in form and 

 size to the nerve-fibre. It is homogeneous, closed, and of uniform thick- 

 ness. (10) It is constricted with the fibre at the intermediate discs, but 

 this is hardly noticeable in young fibres. The more the medulla in- 

 creases, the more Schwann's sheath grows, except at the intermediate 

 discs where there is no medulla. Kanvier's constrictions have no 

 influence on the sheath. 



(11) The axial cylinder has the form of a more or less regular and 

 continuous cylinder. (12) It exhibits an outer, firmer and thinner, an 

 internal softer portion. (13) The outer "cortex" is very pliable, some- 

 what elastic, very fine. It swells in water, becomes for a short time 

 more visible in dilute acetic acid. (14) The content is probably a 



* Arch. f. Mikr. Anat., xxx. (1887) pp. 435-94 (1 pi.). 



