1 68 HUMAN PHYSIOLOGY. 



The reflex excitability of the cord may be 



1. Increased by disease of the lateral columns, by the administration of 

 strychnin, and, in frogs, by a separation of cord from the brain, the latter 

 apparently exerting an inhibitor influence over the former and depressing 

 its reflex activity. 



2. Decreased by destructive lesion of the cord e. g., locomotor ataxia, 

 atrophy of the anterior cornua the administration of various drugs, and, 

 in the frog, by irritation of certain regions of the brain. When the cere- 

 brum alone is removed and the optic lobes are stimulated, the time elapsing 

 between the application of an irritant to a sensor surface and the resulting 

 movement will be considerably prolonged, the optic lobes (Setschenow's 

 center) apparently generating impulses which, descending the cord, retard 

 its reflex movement. 



Classification of Reflex Movement. They may be divided into four 

 groups, according to the route through which the afferent and efferent 

 impulses pass: 



1. Those normal reflex acts (e. g., deglutition, coughing, sneezing, walking, 

 etc.) and pathologic reflex acts (e. g., tetanus, vomiting, epilepsy) which 

 take place both afferently and efferently through spinal nerves. 



2. Reflex acts which take place in an afferent direction through a cerebro- 

 spinal sensor nerve, and in an efferent direction through a sympathetic 

 motor nerve e. g., the normal reflex acts, which give rise to most of the 

 secretions, pallor of the skin and blushing, certain movements of the iris, 

 certain modifications in the beat of the heart; the pathologic reflexes, 

 which, on account of the difficulty in explaining their production, are 

 termed metastatic e. g., ophthalmia, coryza, orchitis, which depend on a 

 reflex hyperemia; amaurosis, paralysis, paraplegia, etc., due to a reflex 

 anemia. 



3. Reflex movements in which the afferent impulse passes through a sym- 

 pathetic nerve, and the efferent through a cerebro-spinal nerve; most of 

 these phenomena are pathological e. g., convulsions from intestinal irrita- 

 tion produced by the presence of worms, eclampsia, hysteria, etc. 



Laws of Reflex Action (Pfliiger). 



1. Law of Unilaterality. If a feeble irritation be applied to one or more 

 sensory nerves, movement takes place usually on one side only, and that 

 the same side as the irritation. 



2. Law of Symmetry. If the irritation becomes sufficiently intense, motor 

 reaction is manifested, in addition, in corresponding muscles of the oppo- 

 site side of the body. 



