116 THE ANATOMY OF THE HORSE. 



composed of lymphoid tissue, and included between the layers of the 

 anterior mediastinum. It steadily atrophies after birth, and in the 

 adult only the shrivelled remains of it will be found. 



THE NERVES AND VESSELS OF THE RIGHT SIDE OF THE THORAX 

 (plates 25 AND 26). 



Directions. — Reverse the position of the thorax, turning the right 

 side upwards, and proceed as already directed for the display of the 

 structures on the left side (page 108). 



The Right Phrenic Nerve. This nerve enters the chest by passing 

 beneath the right axillary artery, being included between that vessel 

 and the anterior vena cava. In the anterior mediastinum it lies on the 

 side of the anterior vena cava. It crosses the pericardium as on the left 

 side, and behind the heart it passes across or below the posterior vena 

 cava to reach the diaphragm, where it terminates. Behind the heai't the 

 nerve and the vena cava are included between the layers of a special 

 fold of pleura which passes upwards from the diaphragm and floor of the 

 chest to envelop them. 



The Right Vagus. Separating from the cervical cord of the 

 sympathetic, the right vagus enters the chest by passing under the 

 arch of the right axillary in company with a cai'diac nerve, having 

 the anterior vena cava below. It is then directed obliquely back- 

 wards and upwards across the trachea; and crossing to the inner 

 side of the great vena azygos, it divides at the line of contact of the 

 gullet and windpipe. Each branch unites, as ah*eady described, 

 with the con-esponding branch of the left vagus, thus forming the 

 superior and inferior oesophageal nerves. The thoracic bi'anches of 

 the right vagus are : — 



1. Branches of Communication Avith the middle and inferior cervical 

 ganglia of the sympathetic. 



2. The right Inferior (recurrent) Laryngeal. — This nerve differs from 

 the left in its relations and point of origin. It is given off from the 

 vagus at the origin of the dorso-cervical artery. Turning round behind 

 the root of this trunk, between it and the trachea, it passes forwards 

 on the lower face of the windpipe, above the cephalic artery, and internal 

 to the middle cervical ganglion of the sympathetic. Reaching the root 

 of the neck, it crosses between the carotid artery and the trachea, and 

 is continued up the neck below the artery. In the larynx it is distri- 

 buted in the same manner as the left. In the chest it communicates 

 with the cardiac nerves and with the middle cervical ganglion of the 

 sympathetic, and emits tracheal and oesophageal filaments as on the left 

 side. The right recurrent nerve, it will be observed, is considerably 

 shorter than the left, having its origin at the posterior edge of the 1st 

 rib, while the left has its origin at the base of the heart. Moreover, the 



