STRUCTURE OF THE MEDULLA OBLOXGATA. 509 



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 lamina) by the white 

 or fibrous substance which is arranged partly in external 

 columns, and partly in fasciculi traversing the central grey 

 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 



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. 



