Nervous System 



347 



nating layers of myelin and protein; sur- 

 rounded by the thin protoplasm of the sheath 

 cells of Schwann (5), arrayed in tandem and 

 (in medullated fibers) corresponding each to 

 an internodal segment (between two nodes of 

 Ranvier) ; the whole enclosed in a collage- 

 nous tube (t). Axons vary in diameter (cali- 

 ber) according to the classes to which they 

 belong (sympathetic, somatic motor, tactile 

 sensory, etc.) and, within each class, accord- 

 ing to length and peripheral distribution; 

 the thickness of the myelin sheath and usu- 



different types of neurons differ substantially 

 in their constitution (molecular content), as 

 evidenced by selective reactions to poisons, 

 drugs, histological stains, hormones, and the 

 production of specific secretions (neurosecre- 

 tions). This specificity of neurons may be 

 assumed to extend into the axis cylinders, 

 which are true protoplasmic extensions of the 

 cell. Many more items could be added to 

 this list. In their totality, they make up what 

 may be described as the "functional architec- 

 ture" of the CNS. 



Fig. 125. Diagram of composition of single nerve fiber (explanation in text). 



ally also the length of internodes vary pro- 

 portionately. These parameters are impor- 

 tantly related to functional properties (con- 

 duction velocity, threshold, susceptibility, 

 etc.). Fibers remain single or branch out, 

 either by terminal flaring or by side shoots 

 (collaterals), in accordance with functional 

 needs. Each branch ends either blindly or 

 on a specific end-organ with which it forms 

 transmissive and trophic connections ("in- 

 nervation"). Nerve fibers of a given class 

 generally are found connected with the 

 appropriately matching type of end-organ. 

 The density of innervation varies regionally 

 and with the type of innervated organ. 



THE CENTRAL NERVOUS 

 SYSTEM (CNS) 



The CNS and its outpost ganglia contain 

 the cell bodies (perikarya) of the peripheral 

 neurites, and the former also a host of intra- 

 central fibers, either in orderly bundles 

 (fiber tracts, funiculi, commissures) or ir- 

 regularly dispersed as "neuropil." In addi- 

 tion, these neural masses are interlaced with 

 various types of glia cells, blood vessels, and, 

 according to some, ground substance of ill- 

 defined nature. Morphologically, the cell 

 bodies vary characteristically in size, loca- 

 tion, grouping, number, distribution of proc- 

 esses, and mode of interconnections, all of 

 which may be called "geometrical" criteria. 

 In addition to these distinctions, however. 



NEUROGENESIS 



Every feature thus singled out raises a 

 separate question as to its origin. We have 

 noted the finished products; but how have 

 they come about? This circumscribes the 

 task of neurogenesis. Our goal is to recon- 

 struct the whole causal chain of events that 

 leads from the properties of the egg cell to 

 each particular item on our list. These events 

 include complex molecular interactions with 

 the emergence of new molecular species and 

 the loss of others; displacements and rear- 

 rangements of substance on a molecular, 

 micellar, cellular, and supracellular scale; 

 metabolic energy production and consump- 

 tion; electric, thermal, hydrodynamic, and 

 mechanostatic (pressure-tension) phenomena, 

 etc.; at any rate, processes that are in prin- 

 ciple observable, measurable, and describ- 

 able in terms of the substances, forces, inter- 

 actions, and conditions actually present at 

 each step. This is the object of ontogenetic 

 analysis. Phylogenetic (evolutionary) con- 

 siderations, introducing past history, do not 

 enter into this causal (or operational) analy- 

 sis at all; they only explain why of the in- 

 finite conceivable number of possible causal 

 chains of events, only a very limited selec- 

 tion has found materialization. 



Causal analysis must be preceded by a 

 complete description of the events that are 

 to be explained; description, of course, in 

 objective and, wherever possible, quantitative 



