THE FUNCTION OF THE MEDULLA OBLONGATA 641 



the destruction of the medulla itself is followed by an almost immediate 

 cessation of the respiratory movements, a relaxation of the vascular 

 channels and a stoppage of the heart. 



The centers situated in the domain of the bulb are of two kinds, 

 namely, simple reflex and dominating or automatic. Regarding 

 their function, nothing further need be said, because the manner in 

 which reflex action is effected has already been discussed in detail 

 in an earlier chapter. The following bulbar reflex centers have been 

 localized with some degree of accuracy: 



(a) Closure of the eyelids. The sensory impulses reach the medulla from the 

 cornea, conjunctiva, and vicinity of the eyelids by way of the trigeminus nerve. 

 They are transferred to the motor fibers of that branch of the facial nerve which 

 innervates the orbicularis palpebrarum. The center itself extends from the ala 

 cinera to the posterior border of the pons. While this reflex is bilateral in character, 

 the volitional closure of the lids may be unilateral and may be intensified by the 

 contraction of the neighboring muscles of the face. 



(6) Center for sneezing. The afferent arc is formed by the trigeminus, and the 

 efferent arc by the nerves innervating the different muscles of respiration. In 

 addition, afferent impulses may be received by way of the olfactory and optic 

 nerves, because this reflex is also evoked by intense odors and sudden high intensi- 

 ties of light. 



(c) Center for coughing. It is situated above the center for respiration. The 

 sensory side of this reflex circuit is formed by the afferent fibers of the vagus, and 

 the efferent arc by the nerves innervating the muscles of the larynx and the expira- 

 tory muscles of the thorax. 



(d) Center for mastication and sucking. The sensory path includes the second 

 and third branches of the trigeminus and the glossopharyngeus. The motor 

 path includes the facialis to the muscles of the lips, the hypoglossus to the tongue, 

 and the third branch of the trigeminus to the muscles raising and lowering the 

 lower jaw. 



(e) Center for deglutition. It is situated near the floor of the fourth ventricle 

 above the respiratory center. The afferent side of this circuit is formed by the 

 second and third branches of the trigeminus and the vagus. Its efferent side is 

 formed by the vagus. 



if) Center for the secretion of saliva. It is placed near the floor of the fourth 

 ventricle and may be activated by different sensory impulses. Its efferent fibers 

 enter the parasympathetic system and appear peripherally as the chorda tympani 

 and the auriculotemporal branch of the inferior maxillary division of the trigeminus. 



(g) Center for vomiting. Besides the afferent fibers of the vagus, these im- 

 pulses may also be derived from other sensory tracts, such as the optic and ol- 

 factory. The chief efferent fibers are contained in the vagus. 



The Medulla as an Automatic Center. — The foregoing discussion 

 shows that the reflex centers of the medulla are practically identical 

 with the nuclei of the different cranial nerves concerned in these 

 reactions. For this reason, the latter may be considered as gene- 

 rating a state of nervous activity very similar to that displayed by 

 the spinal nuclei or by the cells of the automatic centers regulating 

 the most vital processes in our body, namely, respiration, the action 

 of the heart, and the distribution of the blood. These functions are 

 of such great importance that the medulla is capable of assuming 

 through them a position almost independent of the cerebrum and 



41 



