998 



HUMAN ANATOMY. 



Fig 



Semidiagrammatic representation of 

 structure of neurone ; a, axone. 



On reaching their destination the axones terminate in end-arborizations (telodendria) 

 of various forms, in a manner similar to the dendrites. According to the distribution 



of their axones, the neurones are divided into two 

 classes. In those of the first, known as cells of type /, 

 the axone is continued as a nerve-tibre and is, therefore, 

 relatively long. Soon after leaving the cell-body such 

 axones give off delicate lateral processes, the collaterals, 

 which, after a longer or shorter course, break up into 

 arborizations ending in relation with other and often 

 remote neurones. Neurones of the second and much 

 less frequent class, cells of type II, possess short axones 

 that are not continued as nerve-fibres, but almost 

 immediately break up into complex end-arborizations 

 or neuropodia (Kolliker), limited to the gray matter. 

 The processes of the sejisory neio'ones, as in the 

 case of those constituting the spinal and other ganglia 

 connected with afferent nerves, are so modified during 

 development (Fig. 839) that later both dendrites and 

 axones arise in common from the 'single robust stalk of 

 an apparently unipolar cell. Branching T-like, one 

 process (the dendrite) passes towards the periphery 

 and the other (the axone) extends to and into the 

 cerebro-spinal axis. 



The nerve-cells, as the bodies of the neurones 

 are called, possess certain structural details in common, 

 although in some instances they present characteristics 

 that sufifice to identify them as belonging to particular localities. Nerve-cells are 

 relatively large elements, those in the anterior horns of the spinal cord measuring 

 from .070- 150 mm. in diameter, and contain a large spherical nucleus, poor in 

 chromatin but usually pro- 

 vided with a conspicuous 

 nucleolus. Their cytoplasm 

 varies in appearance with 

 the method of fixation and 

 staining to such an extent 

 that considerable uncertain- 

 ty exists as to the relation 

 of many described details to 

 the actual structure of the 

 cells. It may be accepted 

 as established, however, 

 that the cell-body of the 

 neurone consists ois^groioid 

 substance, homogeneous or 

 finely granular, in which 

 delicate 7f;5rz7/<^ and masses 

 of chromatophilic granules 

 are embedded ; in addition, 

 a variable amount of brown 

 or blackish pigmejit is com- 

 monly present in the vicin- 

 ity of the nucleus. The 

 presence of the fibrillae 

 within the nerve-cell, long 

 ago maintained by Max 

 Schultze but later disre- 

 garded, has been placed 

 beyond question by the researches of Apathy, Bethe, Cajal and others. The signifi- 

 cance and relations of the fibrillae to the nerve-cell, however, have given rise to warm 



Fig. 838. 



Nerve-cells of human spinal cord stained to show Nissl bodies ; D , dendrites ; 

 /I, axones; C", implantation cone ; A^, nucleus ; W/, nucleolus. X 400. 



