ON THE DEVELOPMENT OF THE ANIMAL TISSUES. 179 



the extremities are shown to be obliquely truncated, where the fasciculi 

 are attached to surfaces not at right angles to their direction. 



" Lastly. He states his opinion, and gives new facts on which it is 

 founded, that in muscular contraction the discs of the fasciculi become 

 approximated, flattened, and expanded ; the fasciculi, of course, at the 

 same time becoming shorter and thicker. He considers that in all con- 

 tractions these phenomena occur; and he adduces arguments to show 

 the improbability of the existence of any rugae or zigzags as a condition 

 of contracting fasciculi in the living body. The paper is abundantly 

 illustrated by drawings of microscopic appearances "] 



2. Nerves. Each entire nervous fibre is to be regarded as a second- 

 ary cell, formed by the coalescence of a series of primary nucleated 

 cells. Schwann is of opinion that the white substance of the nervous 

 fibre, which forms a tube around Remak's band-like axis of the fibre, or 

 the cylindrical axis of Purkinje, is a secondary deposit on the inner 

 surface of the membranous wall of the secondary cell. He finds that 

 the white substance of each nervous fibre is invested externally by a 

 peculiar structureless sheath like that of the primitive muscular fasci- 

 culi. This membranous sheath can be distinguished as a transparent 

 border external to the opaque white substance of the fibre. Its defined 

 outline is opposed, Schwann remarks, to the view of its being composed 

 of cellular tissue. In perfectly formed nervous fibres Schwann some- 

 times perceived here and there at the side of the fibre a nucleus which 

 lay included in the transparent border formed by the membranous 

 sheath. In the grey nervous fibres no white substance is formed. 



[In the substance of the brain of young embryos, Valentin observed 

 cells, on the outer surface of which a granular mass was gradually de- 

 posited. These cells subsequently became nuclei; and their former 

 nuclei became nucleoli ; while the granular matter deposited around 

 them formed the mass of the ganglionic globules, which were thus de- 

 veloped. Valentin has also observed, that after the development of 

 nervous fibres, nuclei, elongated fibre cells, and fully developed fibres of 

 cellular tissue are formed around them. *] 



Schwann's discoveries are to be ranked amongst the most important 

 steps by which the science of physiology has ever been advanced. 



* Compare Valentin's observations on the Development of Tissues in Wagner's 

 Physiology, p. 132. [Willis's translation, p. 214.] Henle's observations on the 

 Structure of the Tissues in his Symbola ad Anatomiam cet. Berol, 1837. Miiller's 

 Archiv., 1838, p. 102. Froriep's Notiz. n. 294. 



