TISSUES, ORGANS, AND SYSTEMS. 109 



cent branchings of muscular fibres, which Corti and I noticed in the 

 tongue of the frog, are on tlie other hand rare, and have been seen else- 

 where only in Artemia salina and in the oral and anal disc of Piscicola 

 (Ley dig).* 



Literature. — W. Bowman, article Muscle and Muscular Motion, in 

 Todd's " CyclopDsdia of Anatomy," and " On the Minute Structure of 

 Voluntary Muscle," in " Phil. Trans.," 1840 II. 1841, I. ; J. Hoist, " De 

 Structurii Musculorum in genere et annulatorum Musculis in Specie," 

 Dorp. 1846; M. Barry, "Neue Unters. iiber die schraubenformige Be- 

 schaifcnheit der Elementarfasern d. Muskeln, nebst Beobachtungen liber 

 die musculos. Natur d. Flimraerharchen" (Miill. "Arch.," 1850, p. 529). 



§ 28. Nervous tissue. — The essential elements of this tissue are of 

 two kinds, the nerve-fibres and nerve-cells {ganglion-globules). The pri- 

 mitive fibres or tubules of the nerves have either a distinct medulla or 

 they have none. The former consist of three parts : of a structureless 

 delicate membrane, the sheath of the primitive tubules ; of a central, 

 soft, but elastic fibre, the central or axis band {axis cylinder, Purkinje ; 

 'primitive band of Remak) ; and of a viscid white layer placed between 

 them, the medullary sheath. In the tubules without medulla, which in 

 man occur only in certain peripheral expansions (retina, olfactory organ, 

 cornea. Pacinian corpuscles), the structureless coat contains nothing but 

 a homogeneous or finely-granulai-, clear substance, which appears to be 

 identical with the central band of the other tubules, and at any rate 

 may be considered analogous to it, so that the medullary layer may be 

 supposed to be absent in these. The primitive nerve-tubules of both 

 kinds, especially of the former, occur of very different dimensions, and 

 may thence be divided into fine ones of 0-0005-0-002 of a line, those of 

 a medium size of 0-002-0-004, and thick ones of 0-004-0-01 of a line. 

 Their course is either isolated, so that one tubule runs from the centre to 

 the periphery; or they divide, especially in their terminal expansions, 

 into a greater or smaller number of branches; or, lastly, they form 

 actual anastomoses and networks. Besides this, many nerve-tubules are 

 connected with nerve-cells, so that they either arise from them or are 

 interrupted in their course by interposed nerve-cells. These nerve-cells, 

 or as they are called in the ganglia, ganglion-cells, ov ganglion-globules, 

 are endowed with the common attributes of cells. Their membrane 

 presents no peculiarity, except that, frequently, it is very delicate, and 

 even, as in the great central masses, eventually perhaps wholly disap- 

 pears. The contents are finely-granulated, semi-solid, often contain 



* [Such branched muscular fibres may be found beautifully marked in the upper lip of the 

 Rat, and in tlie tongue oiMan and Animals. See article " Tongue," by Dr. Hyde Salter, in 

 Todd's "Clycopaidia."— Tks.] 



