THE NERVOUS TISSUES. 



999 



discussion. The observations based upon the improved methods of silver-staining 

 introduced by Cajal have contributed much towards the solution of these questions, 

 and, at present, the most experienced histologists incline towards the view that the 

 fibrillae demonstrable within the nerve-cell are limited to the body and processes of 

 that particular neurone and do not unite with the nbrillae of other neurones. When 

 adequately differentiated by successful staining, the nbrillae form an intracellular 

 net-work within the cell-body, from which they are continued into the dendrites and 

 axone and in all cases end free in the terminal arborizations (Retains;. 



After special staining with methylene blue, or other basic anilines, the chro- 

 matophilic granules appear deeply colored and arranged in groups or masses of vary- 

 ing form and size. Such aggregations, known as Nissl bodies, after the German 

 histologist whose elaborate studies and theories concerning the structure of the nerve- 

 cell have given prominence to these masses of " stainable substance," are usually 

 most conspicuous in the vicinity of the nucleus. Collectively, they constitute the 

 tigroid substance of Lenhossek and are least marked at the periphery of the nerve- 

 cell. They are continued into the dendrites as elongated flakes or pointed rod-like 

 tracts that finally are resolved into scattered granules along the processes. The 

 axone, on the contrary, is not invaded by the Nissl bodies, and usually joins the 

 nerve-cell at an area free from the stainable substance, the axis-cylinder process com- 

 monly arising from a slight elevation known as the implantation cone. Exception- 

 ally, the axone may arise from one of the dendrites, either at its base or at a point 

 some distance from the cell-body. 



Notwithstanding the elaborate classification of nerve-cells and the theories based upon the 

 Nissl bodies, their significance is still debatable, although in the light of the more recent studies 

 by Carrier, Holmes and others it seems probable that they are normal constituents of the cell 

 and are directly related to functional activity, undergoing increase under unusual stimulus. 



Critical study of the structure of ganglionic nerve-cells has established the presence of four 

 fundamental components within their cytoplasm. These are, according to Cowdry, (1) the Nissl 

 bodies, (2) the mitrochondria, deeply staining minute rod-like granules, (3) an intracellular system 

 of clefts or canaliculi, and ( 4 ) the neuro-fibrils. That these canaliculi are not artefacts is probable 

 from their demonstration after staining intra vitam with a solution of pyronin, when the clefts 

 appear as a network of clear, continuous, but tortuous spaces within the red-tinted cytoplasm. 



FIG. 839. 



Every neurone possesses at least one process, which is then an axone, although 

 usually provided with both dendrites and axone. Very 

 rarely more than a single axone is present. Depend- 

 ing upon the number of their processes, nerve-cells are 

 described as unipolar, bipolar, or multipdar. The 

 unipolar condition is often secondary, since two 

 processes may be so blended for part of their course 

 that they form a single process. Conspicuous examples 

 of such relation are seen in the spherical nerve-cells 

 composing the spinal and other ganglia connected with 

 the sensory nerves. Primarily such neurones possess 

 an axone and a dendrite that arise from opposite ends 

 of what is for a time a spindle-shaped bipolar cell. 

 During development, however, the unilateral growth 

 of the cell-body towards the surface of the ganglion 

 brings about the gradual approximation of the two 

 processes until they fuse in the single extension into 

 which the spherical or flask-like cell is prolonged. 

 This process sooner or later undergoes a Y- or T- like 

 division, one process, usually identified as the dendrite, 

 passing to the periphery to end in the free terminal 

 arborization, whilst the other, the axone, passes 

 centrally to end in an arborization around the 

 neurones lying within the cerebro-spinal axis. 



Examples of bipolar neurones, in which the dendrite and axone pass from 

 opposite sides of the spherical cell-body, are found in the retina and the ganglia 



Diagram showing transformation 

 of young bipolar sensory neurone into 

 one of unipolar type. 



