CHAP. VIII.] GELATINOUS FIBRE. 21 L 



apt to form upon some nerve-tubes than upon others; and this is 

 apparently owing to a feebleness of the tubular membrane, and per- 

 haps, also, to a less degree of consistence of the contained nervous 

 pulp. In the nerves of special sensation the tubes are very deli- 

 cate in structure, and very apt to exhibit this change ; and in the 

 fibres of the brain and spinal cord the same tendency is observable. 

 It was formerly supposed by Ehrenberg, that these varicosities 

 were natural, and existed during life; and that they afforded a 

 valuable morphological character of the nerves of pure sense, and of 

 the cerebro-spinal centre. Many circumstances, however, oppose this 

 view; thus, the irregularies in the shape, size, and number of the 

 varicosities appear very unlike a natural disposition : in a piece of 

 the brain or spinal cord, which has not been much pressed or torn, 

 the nerve-tubes often exhibit a cylindrical figure, and even in the 

 manipulated specimen the varicose tubes form only a portion. In 

 some nerves, such as those of muscles, the tubes, although not 

 prone to become varicose, may be made so by firm pressure and 

 violence in manipulation ; and in the nerve-tubes of young animals, 

 the tissues of which are more tender, and contain more abundant 

 water, this change is particularly apt to take place. 



The nerve-tubes, for the most part, lie parallel to each other, 

 always without branching, and, if we except their terminal looping 

 in other textures, without any inosculation. This very interesting 

 and important feature in the anatomy of the nerve-tubes was re- 

 cognised long ago by Fontana, and has been confirmed by nearly 

 every subsequent observer. It may be seen in the nervous centres, 

 as well as in the nerves themselves. In the latter it may be well 

 demonstrated by examining a piece of nerve on a dark ground as 

 an opaque object. The primitive fibres, viewed in this way, appear 

 as so many transparent tubes, containing an exquisitely delicate, 

 soft, pearly-white material. The tubular fibres vary in diameter 

 from T5 -L^ to even 10 00 of an inch ; but their average width is 



from ToVo to 4inro of an incn - 



2. Of the Gelatinous Nerve-fibre. This term is applied by Henle 

 to certain fibres found principally in the sympathetic nerve. They 

 are flattened, soft, and homogeneous in appearance; contain- 

 ing numerous cell-nuclei, some of which are round, others oval ; 

 some situated in the centre of the fibre, others adhering to either 

 edge; their longest diameter being generally parallel to the longitu- 

 dinal axis of the nerve. These nuclei are arranged at nearly equal 

 distances, and frequently exhibit distinct nucleoli (fig. 52, c). Some- 

 times these fibres show a disposition to split into very delicate 



p 2 



