328 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



manner up and down the spinal cord. From the longitudinal portion of 

 the fibre cross-pieces (collateral fibres — Fig. 137, cot) pass off at intervals, 

 which divide up into fine branches in intimate relation with a motor 

 ganglion-cell. The question whether there is actual continuity, of 

 substance between the terminal twigs of the collateral and the cytoplasm 

 of the motor-cell or merely intimate contact is a particular case of one of 

 the most disputed problems regarding the structure of the vertebrate 

 body. While the obvious evidence in the way of observed fact has up 

 to the present failed to give a clear demonstration of such continuity 

 there is still a minority of investigators, including the present writer, 



col, 



-M 



Fig. 137. 



Diagram showing the relations of a spinal nerve to the spinal cord, col, Collateral branch of 

 sensory fibre ; d.r, dorsal root ; ep, epidermis ; M, motor ganglion - cell ; m, muscle - fibre ; 

 m.f, motor-fibre ; S, sensory ganglion-cell ; s,Cf spinal cord ; s./, sensory fibre ; s.g, spinal ganglion ; 

 v.r, ventral root. 



who regard such continuity as probably existing though it may be almost 

 impossible to demonstrate. 



The arrangement of ganglion-cells and nerve-fibres just described is 

 of physiological interest inasmuch as it affords us a glimpse of the type 

 of nervous mechanism concerned in the production of reflex movements 

 — involuntary and immediate responses in the form of movement 

 to particular sensory stimuli. A glance at the diagram will show how 

 a sensory message from the skin may readily influence the motor-cell 

 by way of the collateral and so arouse it to its particular form of 

 activity, that of sending out a motor impulse to bring about contraction 

 of the muscle-fibre. 



