NERVE-TERMINATIONS. 79 



later the dorsal root-fibres of a spinal nerve, grow into the developing cord and enter 

 the immature white matter to end, when development is complete, at various levels in 

 relation with the neurones within the gray matter of the cord. The peripherally directed 

 processes, on the other hand, mingle with the axones from the motor neurones to form 

 the mixed nerves distributed to the various parts of the body. The essential parts of 

 the sensory neurones, the cell-body and the processes, are derivatives of ectodermic 

 elements, while the investing sheaths, whether of the whole nerve-trunk or of the 

 entire ganglion, are contributions of the mesoderm. 



The sympathetic system shares with the spinal ganglia a common origin from 

 the primary ganglion-crest. As the latter differentiates, certain immature cells sepa- 

 rate and migrate ventrally toward the vicinity of the aorta ; along this path of migration 

 later appear groups of cells, foreshadowing the ganglia of the ganglionated cord. 

 Meanwhile, neurones of the spinal cord send axones along the ventral spinal roots and 

 into the developing sympathetic strand, eventually to end around the sympathetic 

 cells and so bring, by way of the white rami communicantes, the sympathetic elements 

 under the influence of the cells of the spinal cord. Before marked segmentation of 

 the ganglionated cord occurs, thickenings appear from which bundles of sympathetic 

 efferents grow mesially to form the prevertebral ganglia ; subsequent migration into 

 the developing viscera establish the terminal ganglia. The neurones of the ganglion- 

 ated cord send axones peripherally, thus contributing the gray rami, which carry 

 sympathetic efferents to the outlying involuntary muscle of the blood-vessels, glands 

 and skin. 



NERVE-TERMINATIONS. 



The terminations of the fibres composing the peripheral nerves the 

 axones of certain motor neurones situated within the cerebro-spinal axis and 

 within the sympathetic ganglia and the dendrites of the neurones within the 

 sensory ganglia supply the apparatus by which the various structures are 

 brought into intimate relation with the central nervous system. Some of 

 these terminations convey impulses that produce various sensations (pain, 

 pressure, muscle-sense, temperature); others transfer impulses resulting in 

 muscular contractions. The nerve-terminations, therefore, may be grouped 

 according to function into sensory and motor endings. 



SENSORY NERVE-ENDINGS. 



Since the sensory endings are the more or less modified terminal arbor- 

 izations of neurones whose cell-bodies lie within the spinal and other sensory 

 ganglia, such terminations are functionally the beginnings of the paths con- 

 ducting sensory stimuli to the central nervous 



system. According to their relations to the sur- /rsni^ /- vo^- 



rounding tissue, the sensory endings are broadly 

 grouped into free and encapsulated. 



Free Sensory Endings. These endings 

 include numbers of nerve-terminations found in 

 the skin and the mucous membranes, chiefly 

 within the epithelium but to some extent also 

 within the connective tissue, and between the 

 fibres of voluntary muscle. As a rule, the sensory Jt e^dTr^fs^Tm^bft ; e "n 5J 

 (afferent) nerve-fibres branch only to a limited ^ .places nerve fibriite tend- 



., . .,-',,.. nate in end-knobs. (Dogiel.) 



degree until near their peripheral destination, 



where they undergo repeated division, always at a node of Ranvier and in 

 various directions. The medullary coat of the main fibre is retained until close 

 to its termination, although some of its branches may course as nonmedullated 

 fibres for a considerable distance before ending or entering the epithelium. 



