206 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



the afferent nerve- Is segmental, the area of skin involved forming a band 

 about the body. ( >n the other hand, the distribution of the proximal branches 

 forming the dorsal roots is such that while part of the axones and their 

 collaterals establish connection with the cord and bulb near the level at which 

 the axone joins it, the principal divisions of the axone often pass along the 

 cord a greater or Less distance in both directions, and thus a long stretch of 

 the cord may receive impulses by way of a single afferent element. 



In some of the lower vertebrates the arrangement of the "efferent" cells 

 is plainly segmental, but in man and the higher mammals this is hardly to be 

 demonstrated. In the least modified parts of the cord, the efferent fibres do 

 arise from cell-bodies mainly within the segment from which the ventral root 

 emerges. But this massing of the efferent cell-bodies is largely obscured by 

 the presence of central cells through the entire length of the ventral horns, 

 while in the portions of the cord controlling the limbs the columns of cells 

 furnishing fibres to a given ventral root may extend through as many as three 

 segments of the cord. The distribution of the efferent fibres is evidently 

 segmental in plan, though highly modified everywhere except in the thoracic 

 cord supplying the portion of the trunk between the limbs. The principal 

 peculiarity in the group of central cells is the great increase in the mass of 

 them as we pass from the cord cephalad, the cerebrum, for example, being 

 composed entirely of central cells. 



Relative Development of Different Parts. — The bulk of the three 

 subdivisions which have been named is very unequal. The central system 

 is far more massive than the afferent and efferent taken together, but the 

 relation cannot be stated with any exactness, since the mass of the peripheral 

 system is not definitely known. 



Connections between Cells. — In determining the connection between 

 cells which permits a nerve impulse in one cell to stimulate another, the fact 

 that the axone is the outgrowth of a cell-body, and that each cell is an inde- 

 pendent morphological unit, forms the point of departure. Under these 

 circumstances the question of the connection between cells takes the more 

 explicit form of the question whether cell-branches may become continuous 

 by secondary union. In several vertebrates there is good histological evidence 

 that such secondary union occurs in a few eases in the central system. 



In one type the axone of one element spreads out and encloses the cell- 

 body of a second after the manner of a cup holding a ball. In other cases 

 it appears that the terminals of a given axone may even penetrate the cell 

 substance of the receiving neurone. 



These an' examples of concrescence. In the majority of eases, however, 

 a close approximation of the parts of two nerve-cells is alone to be seen 

 I Fig. 90). The termination of the discharging axone may be by fibrils or 

 expanded di>ks, and occur either close to or upon the body, dendrites, or 

 even collaterals' of the receiving neurone. If, as seems probable, the 

 dendrites form an important pathway by which the receiving neurone is 

 1 Held: Arehivf. Anat. a. Physiol., Anat. Abthl., Leipzig, 1897. 



