748 THE MEDULLA OBLONGATA. 



THE MEDULLA OBLONGATA. 



The medulla oblongata, which connects the spinal cord with the brain, 

 resembles the cord in many respects, particularly from the fact that it 

 contains centers, which, like those in the spinal cord, convey simple 

 reflexes (for example that of closure of the eyelids). Further, it contains 

 centers, however, that occupy a controlling relation to centers of analo- 

 gous function in the spinal cord; for example the centers controlling 

 the vasomotor nerves, the secretion of sweat, dilatation of the pupils, the 

 reflex movements of the body. With reference to the irritation, some 

 of the centers are reflex, others automatic. 



The normal function of the centers is related to the gaseous inter- 

 change maintained by the normal circulation in the medulla. If this 

 is interrupted by asphyxia or sudden anemia or venous stasis, the centers 

 are, at first, thrown into a state of increased irritation, being later par- 

 alyzed by overstimulation. Overheating also acts as an irritant to the 

 centers. Not all of the centers are active at the same time and they do 

 not all exhibit the same degree of irritability. In the normal body the 

 respiratory and vasomotor centers are in constant rhythmic activity. 

 The cardiac inhibitory center is in some animals not continuously irri- 

 tated ; in others slight stimulation occurs normally only with inspiration 

 (in conjunction with stimulation of the respiratory center). The spasm- 

 center is not irritated at all under normal conditions, and the respiratory 

 center not during intrauterine life. The medulla oblongata is, as the 

 seat of many centers of importance with reference to the maintenance of 

 life, as well as for the transmission of various nerve-paths, of the greatest 

 significance. The details are considered in what follows. The reflex 

 centers will be considered first and then the automatic centers. 



REFLEX CENTERS IN THE MEDULLA OBLONGATA. 



The medulla oblongata contains a number of reflex centers that 

 permit the execution of coordinated movements. 



The center for closure of the eyelids. The sensory fibers of the trigem- 

 inus to the cornea and the conjunctiva, as well as the skin in the vicinity 

 of the eye, conduct centripetally the impressions received to the medulla 

 oblongata, where they are transferred to the motor path of the facial 

 branch that innervates the orbicular muscle of the lids. The center 

 extends from about the middle of the ala cinerea upward to the posterior 

 margin of the pons. 



Intense illumination of the eye also causes closure of the eyelids 

 through the intermediation of the optic nerve. The stimulation passes 

 through the quadrigeminate bodies to the center. 



Reflex closure of the eyelids takes place in man always on both sides, but 

 voluntarily the eyelids can be closed on one side. On intense irritation the cor- 

 ugator and the muscle-group that elevates the nose and the cheek toward the 

 lower margin of the orbit also contract in order to secure more perfect protection 

 and closure of the eye. The duration of voluntary and reflex closure of the eye- 

 lids is from 0.3 to 0.45 second. 



The center for sneezing. The centripetal path is through the inner 

 nasal branches of the trigeminus and probably also through the olfactory 

 nerve (for strong odors). The motor path leads to the expiratory mus- 

 cles. Sneezing cannot be practised voluntarily. 



