QUESTIONS AND ANSWERS 87 



at first in the middle of the posterior face, then to the left side 

 of the trachea. The carotid arteries, the pneiimogastric, great sym- 

 pathetic and recurrent nerves pass along both sides of the tube. 



In the thoracic portion, the trachea is related superiorly to the 

 longus colli muscle and the oesophagus ; below to the brachial vessels, 

 the anterior aorta, anterior vena cava, the cardiac and recurrent 

 nerves, and to the left auricle of the heart ; laterally it is in contact 

 with the inferior cervical ganglia of the great sympathetic, the 

 vertebral vessels and the two layers of the anterior mediastinum, 

 vena azygos, aorta and thoracic duct. 



Describe the thoracic cavity. Name the organs that are contained in 

 this cavity. 



The thoracic cavity is a bony cage, representing a hollow cone 

 placed horizontally, depressed on its sides, apex forward and base 

 formed by the diaphragm, posteriorly. 



Boundaries : in front, the first ribs and longus coUi muscle ; be- 

 hind, the diaphragm ; superiorly, the vertebrae, ribs and longus colli 

 muscle; inferiorly, the sternum, costal cartilages and the triangu- 

 laris sterni muscle ; laterally, the ribs and deep intercostal muscles. 

 Its internal face is covered with a serous membrane, the pleura. 



It contains the following organs : heart and great vessels, lungs, 

 trachea, oesophagus, thoracic duct and lymphatic glands. 



Function: Receptacle for the contained organs; its movements 

 are of primary importance in the act of respiration. 



Compare the thoracic cavity of the horse with that of the ox. 



In the ox, the thorax is not so long, particularly in its superior 

 part, as in the horse, because the diaphragm is situated less obliquely 

 and its attachment to the ribs is more anterior. Its total capacity 

 is less than in the horse. 

 Give the number of puhnonary lobes in (a) the horse, (b) the ox, 

 (c) the sheep, (d) the pig, (e) the dog. 

 (a) 5; (b) 6; (c) 6; (d) 6; (e) 7. 



Describe the lungs of the horse. 



The lungs, the essential organs of respiration, are situated in the 

 thoracic cavity, surrounding the heart, and enveloped by a serous 

 membrane, the pleura. 



Divided into two lateral halves, the lungs are considered as two 

 parts, right and left, which, in contour, correspond to the thoracic 

 cavity which they practically fill. Each half is cone-shaped and 

 has: an external face lying against the ribs; an internal face, 

 forming the side of the mediastinum which presents an excavation 



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