SPINAL CORD. 185 



relation with one another and with the cerebrum and its subordi- 

 nate or underlying parts. 



i. As an Independent Nerve Center. 



The spinal cord, by virtue of its contained nerve-cells, is capable 

 of transforming afferent nerve impulses arriving through the afferent 

 nerves into efferent impulses, which are reflected outward through 

 efferent nerves to muscles, producing motion ; to glands, exciting 

 secretion ; to blood-vessels, changing their caliber. All such actions 

 taking place independent of either sensation or volition are termed 

 reflex actions. The mechanism involved in every reflex action con- 

 sists of a sentient surface, an afferent nerve, an emissive center, 

 an efferent nerve, and a responsive organ, muscle, gland, or blood- 

 vessels. 



The reilex 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 inhibitory influence over the for- 

 mer 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 cerebrum 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) ap- 

 parently generating impulses which, descending the cord, retard 

 its reflex movement. 



Classification of Reflex Movements (Kiiss). They may be di- 

 vided 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 blush- 

 ing, certain movements of the iris, certain modifications in the 



