530 MANUAL OF PHYSIOLOGY. 



THE SPINAL ACCESSORY NERVES. 



These also form part of the eighth pair of nerves, and arise 

 from the oblong and the spinal marrow, as low down as the 

 seventh cervical vertebra. The lower fibres leave the lateral 

 columns at their posterior aspect, and then run up between the 

 denticulate ligament and the posterior roots of the spinal nerves 

 to enter the cranial cavity. On their way out of the cranium 

 they divide into two parts, one of which becomes amalgamated 

 with the vagus, and the other passes down the side of the neck 

 as the motor nerve of the sterno-mastoid and trapezius muscles. 

 Physiologically, it may be compared with the anterior root of a 

 spinal nerve, and the part accessory to the vagus most probably 

 supplies that nerve with most of its motor branches. 



THE VAGUS NERVE. 



The vagus arises from the lower part of the floor of the fourth 

 ventricle, and has connections with many of the important groups 

 of nerve cells in this neighborhood. 



The functions of its widely-distributed fibres may be thus 

 briefly stated : 



A. The EFFERENT FIBRES may be divided into 



1. Motor-nerve channels, going to a great portion of the aliment- 

 ary tract and the air passage ; the following muscles getting 

 their motor supply from the branches of the vagus the pharyn- 

 geal constrictors, some of the muscles of the palate, the oesoph- 

 agus, the stomach, and the greater part of the small intestine. 

 Motor impulses also pass along the trunk of the vagus though 

 leaving the cord by the roots of the accessory nerve to the in- 

 trinsic muscles of the larynx ; these fibres lie in the inferior or 

 recurrent laryngeal nerve except that to the crico-thyroid, which 

 lies in the superior laryngeal branch. The tracheal muscle and 

 the smooth muscle of the bronchial walls are also under the con- 

 trol of the pulmonary branches of the vagus. 



2. Vasomotor fibres are said to be supplied to the stomach and 

 small intestine. These fibres are probably derived from some of 

 the numerous connections with the sympathetic. 



3. Inhibitory impulses of great importance for the regulation of 



