THE NERVOUS SYSTEM. 103 



assume that the cord is morbidly more irritable, i.e., apt to issue more nerv- 

 ous force than is proportionate to the stimulus applied to it, a slight impres- 

 sion on a sensory nerve produces extensive reflex movements. This appears 

 to be the condition in tetanus, in which a slight touch on the skin may 

 throw the whole body into convulsion. A similar state is induced by the 

 introduction of strychnia and, in frogs, of opium into the blood; and 

 numerous experiments on frogs thus made tetanic, have shown that the 

 tetanus is wholly unconnected with the brain, and depends on the state 

 induced in the spinal cord. 



Special Centres in Spinal Cord. It may seem to have been im- 

 plied that the spinal cord, as a single nerve-centre, reflects alike from all 

 parts all the impressions conducted to it. But it is more probable that 

 it should be regarded as a collection of nervous centres united in a con- 

 tinuous column. This is made probable by the fact that segments of the 

 cord may act as distinct nerve-centres, and exeite motions in the parts sup- 

 plied with nerves given off from them; as well as by the analogy of cer- 

 tain cases in which the muscular movements of single organs are under 

 the control of certain circumscribed portions of the cord. Thus, for 

 the governance of the sphincter-muscles concerned in guarding the orifices 

 respectively of the rectum and urinary bladder there are special nerve- 

 centres in the lower part of the spinal cord (ano-spinal and vesico-spinal 

 centres) ; while the actions of these are temporarily inhibited by stimuli 

 which lead to defsecation and micturition. So, also, there are centres 

 directly concerned in erection of the penis and in the emission of semen 

 (genito-urinary). The emission of semen is a reflex act: the irritation 

 of the glans penis conducted to the spinal cord, and thence reflected, ex- 

 cites the successive and co-ordinate contractions of the muscular fibres of 

 the vasa deferentia and vesiculae seminales, and of the accelerator urinae 

 and other muscles of the urethra; and a forcible expulsion of semen takes 

 place, over which the mind has little or no control, and which, in cases 

 of paraplegia, may be unf elt. The erection of the penis, also, as already 

 explained (p. 169, Vol. I.), appears to be in part the result of a reflex con- 

 traction of the muscles by which the veins returning the blood from the 

 penis are compressed. The involuntary action of the uterus in expelling 

 its contents during parturition, is also of a purely reflex kind, dependent 

 in part upon the spinal cord, though in part also upon the sympathetic 

 system: its independence of the brain being proved by cases of delivery in 

 paraplegic women, and also by the fact that delivery can take place whilst 

 the patient is under the influence of chloroform. But all these spinal 

 nerve-centres are intimately connected, both structurally and physi- 

 ologically, one with another, as well as with those higher encephalic 

 centres, without whose guiding influence their actions may become dis- 

 orderly and purposeless, or altogether abrogated. 



Centre for Movements of Lymphatic Hearts of Frog. Volkmann 



