754 THE NERVOUS SYSTEM 



(2) The peripheral nervous sysieiyi, composed of — (a) The cranio-spinal 

 nerves, and (h) the sympathetic nervous system. 



All these parts are so intimately connected with each other that the division is 

 purely arbitrary. The cranio-spinal nerves are anatomically continuous with the 

 central system; their component fibres either arise within or terminate within 

 the confines of the central system, and thus actually contribute to its bulk. The 

 sj-mpathetic system, however, may be more nearly considered as having a 

 domain of its own. By communicating rami, it is intimately associated with the 

 cranio-spinal nerves and thus with the central system, both receiving impulses 

 from the central system and transmitting impulses which enter it. But, while its 

 activities are largely under the control of the central system, it is possible that 

 impulses may arise in the domain of the sympathetic system and, mediated by 

 its nerves, produce reactions in the tissues it supplies without involving the 

 central system at all. For this reason, as well as because of the structural 

 peculiarities of the sympathetic system, the nervous system is sometimes divided 

 into — (1) the cranio-spinal system, consisting of (a) the central system and (6) 

 the cranio-spinal nerves; (2) the sympathetic nervous system, consisting of its 

 various peripheral ganglia and their outgrowths forming its plexuses. 



Within and closely proximal to the central system or axis are grouped the parent cell-bodies 

 whose processes comprise the nerve fibres of the cranio-spinal nerves. Other groups of nerve 

 cell-bodies, distributed in the periphery without the bounds of the central system, give rise 

 to the fibres of the sympathetic nerves and plexuses. Any group of such cell-bodies situated 

 in the periphery, whether belonging to the cranio-spinal or sympathetic system, is known as a 

 ganglion. 



THE DEVELOPMENT OF THE NERVOUS SYSTEM 



The essential elements of the nervous system, the nerve cell-bodies and the 

 essential portion of all nerve fibres, central, cranio-spinal and sympathetic, de- 

 velop from one of the embryonic germ layers, the ectoderm, and all arise from a 

 given region of that germ layer. Further a small portion of the supporting tissue 

 of the nervous system, the neuroglia, is of the same origin. 



In its development the nervous system is precocious. It is the first of the functional 

 apparatuses to begin differentiation and is the first to aci^uire its form. The first trace of 

 the embryo appears on the developing ovum as the embryonic area, and the rapidly proliferating 

 cells of this area shortly become arranged into the three germinal layers: — the outer layer or 

 ectoderm, the middle layer or mesoderm, and the inner layer or enloderm. Early in the process 

 of this arrangement there is formed along the axial line of the embryonic area a thickened plate 

 of ectodermal cells, the neural -plate. In the further proliferation of these cells, the margins 

 of the neural plate, which lie parallel with the long axis of the embryonic area, rise slightly above 

 the general surface, forming the neural folds, and the floor of the plate between the folds under- 

 goes a slight invagination, the process resulting in the neural groove (fig. 595, A, A\ B and B^. 

 As development proceeds and the embryonic area assumes the form of a distinct embryo, the 

 neural folds or lips of the groove gradually converge, and beginning at the oral end, finally unite. 

 Thus the groove is converted into the neural tube, extending along the dorsal mid-line and en- 

 closed within the body of the embryo by the now continuous ectoderm above (fig 595, C^, 

 D and DO- 



For a time the neural tube remains connected with the iimer surface of the general ectoderm 

 along the line of fusion by a residual lamina of ectodermal cells. This lamina is known as the 

 ganglion crest (neural crest). It is a i)roduct of the proliferation of the ectoderm during the 

 process of fusion, consists of the cells which composed the transition between the closing lips 

 of the original groove and the general ectoderm or skin, and whoso fusion aitled in the closure 

 of the tulje. The ectoderm soon becomes .separated from the ganglion crest and the cells of the 

 crest become distinctly difTerentiated from the cells of the neural tube. The essential elements 

 of the entire nervous system together with the neuroglia are dei'ived from the cells of the neural 

 tube and the cells of the ganglion crest. 



) 



Fig. 595. — Dorsal Sukface Views of Human Fmbuyos and Diachams of Tuansvebsb 

 Section.s Illustrating the Development of the Neural Tube. 



A, donsal view of human embryo at beginning of infolding of neural plate to form neural 

 groove. Amnion partly removed. (C.raf Spec, from Keibcl and Mall.) A', diagram of 

 portion of a tran.svense section of an (;inbryo as though taken through A at the line a'. 

 B, dorsal view of hiiiMau cinbryo of 7 somites, neural tube not yot clo.sed, Mall Collection. 

 (Dandy, from KcilKd and Mall.) W, diagram of portion of a transverse sect ion of an embryo 

 as though tak(;n through H at the line b'. C', diagram of portion of a transverse .section of 

 an embryo as though taken through D at line c'. D, dorsal view of human eni})ryo of 8 

 somites, 2.11 m.m. long, neiual tu})0 clo.sed except at caudal end. (Kollmann, from 

 Keibel and Mall.) J)', diagram of a portion of a transverse section of an embryo as though 

 taken through D atline d'. 



