REFLEX ACTIONS. 



149 



The added feature of a psychical factor, a reaction in consciousness, 

 enables us to draw a line of distinction between these activities and 

 those of so-called pure reflexes ; but the distinction is perhaps one of 

 convenience only, for, although the extremes may be far enough 

 apart to suit the definition, many intermediate instances may be 

 found which are difficult to classify. All skilled movements, for in- 

 stance, such as walking, singing, dancing, bicycle riding, and the 

 like, — although in the beginning obviously effected by voluntary 

 co-ordination, nevertheless in the end, in proportion to the skill ob- 

 tained, become more or less entirely reflex, — that is, involuntary. 

 In learning such movements one must, as the saying goes, estabhsh 

 his reflexes, and the result can hardly be understood otherwise than 

 by supposing that the continual adjustment of certain sensory im- 

 pulses to certain co-ordinated movements results in the formation 

 of a more or less complex reflex arc, a set of paths of least resistance. 

 Reflexes through Peripheral Ganglia — Axon Reflexes.— 

 Many attempts have been made by physiologists to ascertain 

 whether or not reflexes can occur through 

 the peripheral nerve ganglia, lying outside 

 the central nervous system. With regard 

 to the posterior root ganglia, it has usually 

 been supposed that they cannot exhibit re- 

 flexes. When the posterior root connecting 

 such a ganglion to the cord is severed, then, 

 according to our usual conception, the cells 

 in the ganglia are cut off from all connec- 

 tions with the peripheral tissues by efferent 

 paths. This usual view may not, however, 

 be correct. On the histological side Cajal* 

 and others have shown that some of these 

 cells are provided with a pericellular nerve 

 network, which is an afferent path so far as 

 the cell is concerned, while the axon of the 

 cell constitutes an efferent path. Whether 

 these cells form a special group of efferent 

 cells lying within the sensory ganghon, or 

 whether they are sensory cells discharging 

 into the cord and stimulated reflexly 

 through the nerve network as well as 

 through the peripheral process of the axon, 

 cannot be said. The subject is one full 

 of interest to physiology. In the ganglia 

 of the sympathetic nerve and its appen- 

 dages and in the similar ganglia contained in many of the organs 



* Cajal, "Ergebnisse der Anat. u. Entwickelungsgeschichte," vol. xvi, 

 1906; Dogiel, "Bau der Spinalganglien, etc.," 1908. 



Schema to 

 Bhow idea of an axon re- 

 flex: The preganglionic 

 fiber, a, sends branches 

 to two postganglionic 

 fibers, b, c. If stimulated 

 at X the impulse passes 

 backward in a direction 

 the reverse of normal and 

 falling into b and c gives 

 a pseudoreflex effect. 



