576 NERVOUS SYSTEM. 



bility, as distinguished from the sense of touch, that we have to study the mode of ter- 

 mination of the nerves. 



Kolliker is of the opinion that, in the immense majority of instances, the sensitive 

 nerves terminate in some way in the hair-follicles. If this be true, it will account for 

 the termination of the nerves in by far the greatest portion of the skin, as there are few 

 parts in which hair-follicles do not exist ; but, unfortunately, the exact mode of connec- 

 tion of the nerves with these follicles is not apparent. The following is all we know 

 positively of the terminations of the nerves on the general surface : 



Medullated nerve-fibres form a plexus in the deeper layers of the true skin, from 

 which fibres, some pale and nucleated and others medullated, pass to the hair-follicles, 

 divide into branches, penetrate into their interior, and are there lost. A certain number 

 of fibres pass to the non-striated muscular fibres of the skin. A certain number pass to 

 papillae and terminate in tactile corpuscles, and others pass to papillaa that have no tac- 

 tile corpuscles. 



In the mucous membranes, as far as we know, the mode of termination is, in general 

 terms, by a delicate plexus just beneath the epithelium, coming from a submucous plexus 

 analogous to the deep cutaneous plexus. In certain membranes, we have already noted 

 the termination in bulbs (corpuscles of Krause). In the cornea, the fibres have been 

 followed more minutely than in any other situation, and the results of recent researches 

 upon this subject are very remarkable. These results are so recent and unexpected, that 

 we are hardly prepared to admit them unreservedly without full confirmation. At 

 present we can only state that the observations of Hoyer, Lipmann, and others, con- 

 firmed in part by Kolliker, seem to show that branching nerve-fibres pass to the nucleoli 

 of the corpuscles of the cornea and to the nucleoli of the cells of the posterior layer of 

 epithelium. 



Structure of the Nerve-centres. 



A peculiar pigmentary matter in the nerve-cells and the surrounding granular sub- 

 stance gives to the nerve-centres a grayish color, by which they are readily distinguished 

 from the white, or fibrous division of the nervous system. Wherever this gray matter is 

 found, the anatomical elements of the tissue are cellular, except in the nerves formed of 

 gray, or gelatinous fibres. Under the general division of nerve-centres, we include, ana- 

 tomically at least, the gray matter of the cerebro-spinal centres, the ganglia of the roots 

 of the spinal and certain of the cranial nerves, and the numerous ganglia of the sympa- 

 thetic system. In these parts are found cells, which constitute the essential anatomical 

 element of the tissue, granular matter resembling the contents of the cells, pale fibres 

 originating in prolongations of the cells, elements of connective tissue, delicate mem- 

 branes enveloping some of the cells, and blood-vessels. The most interesting and im- 

 portant of these structures, in their physiological relations, are the cells and the prolon- 

 gations by which they are connected with the nerves. 



Nerve-cells. Anatomists are now pretty well agreed that the following varieties of 

 cells exist in the nerve-centres and constitute their essential anatomical elements; viz., 

 apolar, unipolar, bipolar, and multipolar cells. Although some have denied the existence 

 of apolar cells, there can be little doubt of their presence in the centres in small numbers, 

 and, as is suggested by Kolliker, they may be nerve-cells in an imperfect state of devel- 

 opment. The nerve-cells present great differences in their size and general appearance, 

 and some distinct varieties are found in particular portions of the nervous system and 

 are probably connected with special functions. 



The apolar cells are simply rounded bodies, with granular contents, a nucleus and 

 nucleolus like other cells, but without any prolongations connecting them with the nerve- 

 fibres. They have been observed in the cerebro-spinal centres, and they always exist in 

 the sympathetic ganglia. Those who deny their existence believe that the poles have 



