FUNCTIONS OF THE CEREBRO -SPIN AL NERVOUS SYSTEM. 557 



(crystalline particles), the optic axes of which are arranged radially to the 

 primitive hand. The cylinder-axis is insoluble in ether, and further differs 

 from the white substance of Schwann in readily staining with carmine. 

 The sympathetic nerves contain some fibres that are of a paler color than 

 those that constitute the majority of the cerebro-spiual nerves, and are fur- 

 ther characterized by the absence of a double contour, and by presenting 

 nuclei in their course, but fine dark-edged fibres are abundant; 1 these are 

 believed by some observers to consist of bundles of minute fibrils. The 

 diameter of ordinary nerve-fibres varies from the T -g' r) th to ou/jo^th inch, 

 but the ultimate fibrils above alluded to do not exceed, according to Dr. 

 Beale, -joth to jo,>VoT)th ^' an i ncn - On careful treatment with various 

 hardening solutions, the latter may be isolated, but soon imbibe water and 

 present varicose enlargements (Fig. 189). These increase in size till the 

 film becomes unrecognizable and disappears. The precise mode in which 

 the nerves terminate peripherally, can scarcely be said to be accurately 

 known in any instance perhaps, with the exception of the Paciniau corpus- 

 cles. In the case of the motor nerves and of the nerves of special sense, 

 considerable differences of opinion exist (which will be hereafter more par- 

 ticularly referred to), as to whether there is a terminal plexus, or whether 

 the nerves end in loops or in free extremities, or become continuous with 

 gland or muscle-cells. Dr. Klein, in following out the nerves of the Cornea, 

 seems to have satisfied himself, that in this part at least, the fibres end in 

 free extremities, which run up between the cells. Nerve-fibres are divided 

 into afferent or centripetal, which convey impressions to a nervous centre or 

 ganglion ; efferent or centrifugal, which conduct impressions from a centre; 

 and central or iutra mesial Xerves. The Centripetal nerves are subdivided 

 into the sensory nerves and excito-motor nerves, the former conveying im- 

 pressions which are usually perceived or felt by the individual, whilst those 

 conveyed by the latter are often unperceived. The Centrifugal fibres are 

 subdivided into motor nerves, distributed to muscles, both striated and un- 

 striated, secretory fibres distributed to glands, and inhibitory nerves, which 

 are capable of arresting or preventing the contraction of muscular tissue. 

 The existence of fibres capable of causing active elongation of muscular 

 fibre is still sub judice. Nerve-cells vary much in their form, size, and 

 structure. The simpler kinds are round or oval, the larger polygonal and 

 stellate, and it is probable that all possess a nucleus, and give off one, two, 

 or more prolongations, that either intercommunicate with the processes of 

 other cells, or become continuous with the cylinder-axis of a nerve fibre 

 (Fig. 190), gradually acquiring a neurilemma, and the white substance of 

 Schwann. In some instances, as in the cells of various parts of the spinal 

 cord (Fig. 191.), the subdivision and ramification of the prolongations, which 

 are sometimes called protoplasmic processes, are very remarkable, a close 

 network being formed which pervades the adjoining portion of the nervous 

 tissue. In other instances, only one or two processes can be traced, the cells 

 being then termed uni- and bipolar. In the sympathetic ganglia of the 

 frog and other animals Dr. Beale, whose researches have been fully con- 

 firmed by subsequent inquirers, has observed nerve-cells of great complex- 

 ity of structure, the chief microscopic features of which are shown in Fig. 

 192, A. < Such cells are pyriform and nucleated, and from their pointed ex- 

 tremity two fibres arise (a, b), which, on reaching the nervous bundle to 

 which they are distributed, run in opposite directions, thus affording some 

 support to the view maintained by Dr. Beale, that every nerve-fibre, how- 



1 See S. Maver, Art. Sympathetic System of Nerves, in Strieker's Histology, vol. 

 ii, p. 552. 



