692 



A TEXTBOOK OF PHYSIOLOGY 



known as. the principal nucleus (Fig. 419). The descending fibres 

 (corresponding to the comma tract of the cord) are longer, and pass 

 into a tract of fibres known as the fimiculus solitarius (Fig. 419). 

 In this tract also run corresponding fibres from the ninth nerve, and 

 the intermediate nerve of Wrisberg. 



The efferent fibres ariie chiefly from the so-called micleus ambiguus 

 (Fig. 419), and also from the upper part of the same nucleus as the 

 eleventh nerve. The afferent sensations brought up by the tenth 

 nerve are concerned with the respiratory and circulatory systems. 

 Impulses from the superior laryngeal nerve inhibit inspiration, and 

 bring about expiration and coughing. Those from the lung alveoli 



Optic-., 

 Chiasma 



Optic 

 Tract 



Crus 

 Cerebri 



(Olfactory Bult>) 



IL 



(Optic Nerve) 





XII 



"j-'ic. 420. ANTKBO-]NFEBIOB VIEW OF THE CBUBA, PONS, AKD BULB (DIAGKA.M.MATIC), 



TO ILLUSTRATE THE SUPEBFICIAL OBIGIN OF THE CRANIAL NEBVES. 



seguJnte the depth and frequency of inspiration, and possibly also of 

 expiration (see p. 296). Those from the heart (depressor nerve), 

 which generally run in the vagus, go to the vaso-motor centre, and 

 reflexly bring about a fall of arterial pressure, owing to vaso-dilata- 

 tion especially in the splanchnic area. Other fibres of the vagus 

 have a pressor effect, and cause a rise of arterial pressure. Central 

 stimulation of this nerve also brings about reflex inhibition of the 

 heart. 



The effector functions of the vagus nerve may be summarized ax 

 inotor to the levator palati, "the constrictors of the pharynx, tin' 

 muscles of the larynx, and to the smooth muscle of the bronchi 

 and bronchioles, to the muscles of the walls of the oesophagus, stomach, 



