SPINAL CORD. 481 



diminution of the reflex power of the cord may be caused by 

 opium, by chloroform, and by some other substances, while the 

 reflex action is increased by strychnin. If under the skin of the 

 decapitated frog a solution of strychnin is injected, the cord in a 

 short time becomes so irritable that a stimulus which before would 

 have had no effect will now produce the most marked results, a 

 slight blow upon the skin sufficing to throw the animal into a con- 

 vulsive state. In tetanus the same irritable condition of the cord 

 exists, and in this state the patient may be thrown into convulsions 

 by the simple opening and closing of a door. 



Special Centers in the Cord. It is the practice to speak 

 of certain centers as existing in the spinal cord that is, of 

 definite collections of cells which preside over definite functions. 

 Among these centers the following have been described : Musculo- 

 tonic, respiratory, cardio-accelerator, vasomotor, sudorific, cilio- 

 spinal, genitospinal, anospinal, vesicospinal, trophic, for erection 

 of the penis, for parturition, and others. 



Musculotonic Center. This center is continually discharging 

 impulses which keep the muscular system in a condition of slight 

 contraction : this is called muscular tone. It is questionable 

 whether this condition is to be attributed to any special center 

 rather than to the action of all those cells whose function it is to 

 send out motor impulses. 



Respiratory Center. The respiratory center is in the medulla, 

 but experiments in which this structure has been destroyed while 

 some respiratory movements persisted demonstrate that to a cer- 

 tain extent, doubtless very slight, the spinal cord controls the 

 respiratory processes. 



Cardio-accelerator Center. The spinal cord through the cardiac 

 nerves and plexus sends impulses to the heart, causing it to beat 

 more rapidly that is, they accelerate its movements. These 

 impulses are not constantly emitted as are the inhibitory impulses, 

 which travel by the pneumogastric. 



Vasomotor Center. The vasomotor center in the cord is entirely 

 subsidiary to that in the medulla. 



Sudorific Center. The existence of special nerves controlling 

 the secretion of sweat seems to be demonstrated. These nerves 

 come from the spinal cord, being a part of the anterior roots. 



Ciliospinal Center. Nerve-fibers pass from this center to the 

 iris, and they are concerned in the dilatation of the pupil. These 

 fibers come out from the cord through the anterior roots of the 

 spinal nerves, from the fifth cervical to the fifth thoracic, and 

 join the cervical sympathetic. 



Genitospinal Center. The genitospinal is the center which 

 governs the emission of semen, and is situated in the lumbar 

 region of the cord. Sensory impulses from the glans penis reach 

 this center through afferent nerves and stimulate it, and from it 



