402 THE NERVOUS SYSTEM. 



in permanent contraction, so that, except in the act of defeca- 

 tion, the orifice of the anus is always closed. This influence 

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

 untary, although the will can act on the muscle to make it 

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

 stant action of the muscle is not felt, nor diminished in sleep, 

 nor productive of fatigue. But the act is different from ordi- 

 nary reflex acts in being nearly constant. In this respect it 

 resembles that condition of muscles which has been called 

 tone, 1 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 mus- 

 cles of the trunk and limbs seems to depend on the spinal 

 cord, as the contraction of the sphincter ani does. If an ani- 

 mal 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 gray 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 gray substance 

 which it contains is situated in the interior, and variously di- 

 vided into masses and laminae by the white or fibrous substance 

 which is arranged partly in external columns, and partly in 

 fasciculi traversing the central gray matter. The medulla 

 oblongata is larger than any part of the spinal cord. Its 

 columns are pyriform, enlarging as they proceed towards the 

 brain, and are continuous with those of the spinal cord. 



Each half of the medulla, therefore, may be divided into 

 three columns or tracts of fibres, continuous with the three 



1 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. 



