510 THE NERVOUS SYSTEM. 



of the cord resembles its common reflex action in being in- 

 voluntary, although the will can act on the muscle to make 

 ft contract more or to permit its dilatation, and in that the 

 constant action of the muscle is not felt, nor diminished in 

 sleep, nor productive of fatigue. But the act is different 

 from ordinary reflex acts in being nearly constant. In 

 this respect it resembles that condition of muscles which 

 has been called tone,* or passive contraction ; in a state in 

 which they always appear to be when not active in health, 

 and in which, though called inactive, they appear to be in 

 slight contraction, and certainly are not relaxed, as they 

 are long after death, or when the spinal cord is destroyed. 

 This tone of all the muscles of the trunk and limbs seems 

 to depend on the spinal cord, as the contraction of the 

 sphincter ani does. If an animal be killed by injury or 

 removal of the brain, the tone of the muscles may be felt, 

 and the limbs feel firm as during sleep ; but if the spinal 

 cord be destroyed, the sphincter ani relaxes, and all the 

 muscles feel loose, and flabby, and atonic, and remain so 

 till the rigor mortis commences. 



THE MEDULLA OBLONGATA. 



Its Structure. 



The medulla oblongata is a mass of grey and white 

 nervous substance partly contained within the cavity of the 

 cranium, -forming a portion of the cephalic prolongation 

 of the spinal cord and connecting it with the brain. The 

 grey substance which it contains is situated in the interior, 

 and variously divided into masses and laminse by the white 

 or fibrous substance which is arranged partly in external 



* This kind of tone must be distinguished from that mere firmness 

 and tension which it is customary to ascribe, under the name of tone, to 

 all tissues that feel robust and not flabby, as well as to muscles. The 

 tone peculiar to muscles has in it a degree of vital contraction : that of 

 other tissues is only due to their being well nourished, and therefore 

 compact and tense. 



