118 THE ANAT0M4" OF THE HORSE. 



composed of tissue resembling that of a lymphatic gland, and 

 included between the layers of the anterior mediastinum. It steadily 

 atrophies after birth, and before the period of adult life is reached 

 it has almost or entirely disappeared. 



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 110). 



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 

 heart 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 cardiac nerve, having 

 the anterior vena cava below. It is then directed obliquely backwards 

 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 already described, with the cor- 

 responding branch of the left vagus, thus forming the superior and 

 inferior oesophageal nerves. The thoracic branches of the right vagus 

 are : — 



1. Branches of Communication with 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 posrorior edge of the 1st 



