DEVELOPMENT OF THE NERVOUS SYSTEM. 



1009 



Fig. 854. 



and has traced the nonmeduUated fibres along the dorsal roots into the spinal cord. 

 The presence of fibres probably derived from sympathetic neurones has been 

 demonstrated. 



The sympathetic ganglia are represented by those of the great gangliated cords, 

 certain cranial ganglia (ciliary, spheno-palatine, otic, and submaxillary), the ganglia 

 within the three prevertebral plexuses, and the innumerable small and often micro- 

 scopic ganglia associated with the muscular tissue of the digestive, respiratory and 

 uro-genital tracts, in the heart and in the various glands. 



In their general structure the sympathetic ganglia are similar to those connected 

 with the spinal nerves, forming definite masses enclosed by a fibrous capsule, from 

 which connective-tissue processes pass into the interior of the ganglion for the support 

 and separation of the nervous 

 elements. The individual gangli- 

 on-cells — unipolar, bipolar or multi- 

 polar — are ensheathed by nucleated 

 capsules continuous with the neuri- 

 lemma of the nerve-fibres. The 

 sympathetic ganglion-cells are vari- 

 ously related to the terminal ramifi- 

 cations of (a) other sympathetic 

 neurones and of (<^) the neurones 

 of the central nervous system (by 

 way of the white rami fibres or their 

 equivalents). In both cases, the 

 ramification of the nonmeduUated 

 and fine fibre in the one and of the 

 meduUated fibre in the other, a 

 pericellular plexus, commonly en- 

 closes the cell-body. In the lower 

 vertebrates (amphibians and reptiles), the spinal fibre frequently winds spirally around 

 the single process of the ganglion-cell before breaking up into the pericellular plexus 

 (Huber*). The broader relations of the component nervous elements of "the spinal 

 ganglia are considered in connection with the Sympathetic System (page 1354). 



Diagram of constituents of spinal ganglion ; blue lines repre- 

 sent efferent fibres ; black, afferent ; red, sympathetic ; a, sensory 

 ganglion cells; c, cells of type II, whose axones end (A) around 

 sensory cells; rf, sympathetic neurone; AR, PR, anterior and 

 posterior roots; AD, PD, anterior and posterior primary divi- 

 sions of spinal nerve; RC, ramus communicans. 



DEVELOPMENT OF THE NERVOUS TISSUES. 



Reference to the account of the early development of the nervous system (page 26) will 

 recall the fact that the neural groove, later the neural tube, is lined by invaginated and thickened 



ectoblast from which the essential nervous tissues are 

 derived. For the fundamental facts concerning the histo- 

 genesis of these tissues we are in large measure indebted 

 to the labors of His, whose account, supplemented by the 

 important contributions of Kolliker, Cajal, Lenhoss^k, 

 Schaper and others, forms the basis of our knowledge 

 concerning these processes. Although in its principal 

 features the histogenesis is similar in all parts of the 

 neural tube, in that portion which becomes the spinal 

 cord the changes are most typical and will, therefore, be 

 here described. 



During the approximation and closure of the neural 

 tube the cells composing its wall undergo active prolife- 

 ration, whereby the wall, at first composed of only one 

 or two rows of definitely outlined cells, is converted into 

 a multinucleated tract in which the cell boundaries dis- 

 appear and the nuclei lie embedded within a general 

 protoplasmic sheet or syncytium (Hardesty^). The 

 large dividing elements within the latter, the germinal 

 cells of His, are conspicuous on account of their mitotic 

 figures and are situated close to the lumen of the neural tube. His regarded them as special 

 cells directly concerned in the production of the neurones, a conclusion, however, that has not 



1 Journal of Morpholog\% 1S99. 



^Amer. Journal of Anatomy, vol. iii., 1904. 



64 



ilm 



Segment from lateral wall of neural tube 

 of pig embryo of 5 mm. ; syncytium replacing 

 distinctly outlined cells, a, inner zone; g, 

 germinal cells; ilm, internal limiting mem- 

 brane ; ni, peripheral zone ; r, radial strands 

 of cytoplasm. X 690. {Hardesty.) 



