NERVE REFLEXES IN ANIMALS 261 



movements to a marked extent. As the spinal cord expands into the brain proper, so its substance and function 

 reappear in an exalted form in the latter organ. This explains why certain involuntary nerve acts called reflex 

 crop up amongst the volitional or mental acts proper, and how the voluntary acts become involuntary if repeated 

 at intervals and frequently. 



Musical performances, which at first are the result of laborious voluntary effort, ultimately become involuntary 

 and automatic. The so-called involuntary movements are the harbingers or forerunners of the vohtional or volun- 

 tary, and it is very difficult, if not indeed impossible, to draw a hard and fast hne between them. It must never 

 be forgotten that what are called automatic, instinctive, involuntary movements in the lower animals are, at the 

 outset, intellectual, and intentional on the part of the Creator. 



Reflex action is, in some respects, a very complicated, and extensive subject. Its ramifications involve both 

 the cerebro-spinal and sympathetic systems of nerves. The cerebro-spinal system and ganglia, as explained, preside 

 over the voluntary muscles, and the sympathetic system and ganglia over the involuntary ones. As the reflex 

 nerve centres are connected with both the cerebro-spinal and sympathetic systems of nerves, their actions may be 

 either conscious or unconscious. 



Reflex action may be simple or compound, according to the number of parts affected by it. Thus one or 

 several muscles may come under its influence, and so of glands and numerous other structures. A reflex centre 

 may consist of a single gangUon with a nucleus, nucleolus,iand a hmited number of nerve cells (uni-polar, bi-polar, and 

 multi-polar), or of several ganglia and a large number of nerve cells. In the latter case, the ganglia are connected 

 and have complicated relations, their action being far reaching. Thus in a frog with the brain and medulla 

 oblongata destroyed, tickhng of the sole of one foot produces movements in the muscles of that foot and the leg 

 to which the foot belongs. If the tickling be increased, the sensation engendered may pass across to gangUa on 

 the other side of the spinal cord and produce movements of both feet and both legs. If pushed still further, it 

 may produce movements of the superior extremities, and even of the whole body, in which case the frog is convulsed. 



The reflex nerve centres are composed chiefly of grey matter ; the sensory and motor nerves connected with 

 them consisting of white matter. 



The following are a few of the accepted tenets explaining the nature of reflex action according to the modern 

 mechanical school of physiology : — 



" Reflex actions may terminate either in movement of a muscle or in secretion of a gland. A reflex move- 

 ment, of the most simple type, may be thus described : (1) excitation of a sensory or afferent nerve ; (2) excitation of 

 an intermediate nervous or reflex centre ; and (3) excitation of a motor or efferent nerve which causes the muscular 

 contraction." 



" The initial excitation may occur both in nerves of general sensibility and in those of the special senses ; but 

 certain nerves more easily excite reflex actions than others." 



" A reflex movement may occur whether we excite a sensory nerve at its commencement or at some point in 

 its com-se, but in the latter case the action is less intense than in the first." 



" The excitability is increased when these centres are severed from communication with psychical centres which 

 preside over voluntary movements." 



" Reflex movements may occur in one muscle, or in many muscles or groups of muscles. Thus they may be simple 

 or compound, and when compound there may be contractions of muscles occurring simultaneously or successively." 



" Reflex centres may be so arranged in the body as to constitute a series in which those in the cerebrum govern 

 or control others in the deeper gangUa of the brain, while these in turn have an influence over still lower centres 

 in the spinal cord. This arrangement has been termed the super-position of reflexes." 



" It is possible for the same stimulus to produce a reflex action of movement, a reflex action of secretion, 

 and also a conscious perception." 



" In compoimd reflex actions, the initial excitation may occur in psychical centres." ^ 



Several of the statements here made are more or less contradictory. 



According to prevaiKng views reflex action has its origin or mainspring in irritability and extraneous stimu- 

 lation. I am wholly opposed to this explanation of reflex action, as neither the irritabihty nor the stimulation 

 can be detected in a healthy normal animal. These appear only in the diseased or mutilated animal. Irritability, 

 extraneous stimulation, and reflex action are further regarded as the cause of all the involuntary, as contra-distin- 

 guished from the voluntary, acts of the body. Here again I am at issue with modern physiologists. The involuntary 

 acts are no more dependent on irritabihty and stimulation than are the voluntary ones. As a matter of fact the 

 ganglia or nerve centres of the cerebro-spinal and sympathetic systems of nerves, with their afferent and efferent 



' " Outlines of Physiology in its Relatioas to Man," by Jolm (.(ray McKendrick, M.D., F.R.S.E,, Professor of Physiology in tlie University of 

 Glasgow. 



