THE SPINAL CORD 



191 



It appears that consciousness tends to prevent or reduce the number of 

 reflex movements. Thus, the tickling of fhe feet during sleep will cause 

 muscular contraction more readily than when we are awake. And again, the 

 unexpected motion of an object near the eyes causes the eyelids to close 

 instantly ; whereas, had we been prepared for it, we should probably have 

 been able to keep the eyes open. 



It has been ascertained that the majority of the motor fibres, which 

 originate in the brain, pass down to the front of the medulla, and then cross 

 over, and descend in the cord on the opposite side, while the remaining 

 fibres descend the cord on the same side. Hence, if one lateral half of the 

 cord is injured, the power of voluntary movement will be almost entirely lost 

 on the same side of the body, below the seat of the injury, but will be 

 practically unimpaired on the opposite side. The sensory fibres, on entering 

 the cord, pass chiefly up the same side to the medulla, and then cross over to 

 the other side of the brain, while some cross to the opposite side immediately 

 on entering the cord. This being the case, injury to one lateral half of the 

 cord will cause impaired sensation chiefly on the same side of the body. 



THE 

 SPINAL 



COED 



Position . 



Length . 

 Thickness 



Coats . 



Reflex action 



SUMMARY. 



j In the spinal canal formed by the vertebral 

 1 column. 



About eighteen inches. 



Extends from the medulla oblongata to the 

 first lumbar vertebra. 



About one-third of an inch. 



Pia mater delicate, vascular. 



Arachnoid serous, forming a loose bag. 



Dura mater tough, fibrous. 



(A layer of fatty tissue lies between the 

 dura mater and the bony canal. ) 



(Grey matter internal. 

 White matter external. 

 A nterior fissure. 

 Posterior fissure deeper and narrower. 

 Central canal microscopic. 

 t Thirty-one pairs between the vertebrae. 

 I Crowded at the lower extremity of the 



cord cauda equina. 



Nerves . . j Each trunk formed by the union of two 

 | roots. 



I Anterior motor. 

 1 I Posterior sensory. 



(Conduction of impressions. 

 A centre for reflex action (grey matter). 

 Sensory fibres cross as they enter the cord. 

 Motor fibres cross in the medulla ob- 

 longata. 



A disturbed condition of sensory fibres 



Conducted to a nerve centre 



Reflected by the nerve centre along motor fibres to 



their terminations in muscles 

 Muscular contraction. 



Roots 



