CAVITY OF THE THORAX. 305 



CHAPTER IT. 



DISSECTION OF THE THORAX. 



SECTION I. 



CAVITY OF THE THORAX. 



THE cavity of the thorax is the space included by the spinal column, 

 the sternum and ribs, and by certain muscles in the intervals of the bony 

 framework. In it the organs of respiration, and the heart with its great 

 vessels are lodged : and through it the gullet, and some vessels and nerves 

 are transmitted. 



Dissection. When the soft parts covering in front the bony parietes of 

 the thorax have been examined and taken away, the cavity is to be opened 

 by removing a portion of the anterior boundary. To make a sufficient 

 opening in the thorax, the sternum is to be sawn through opposite the in- 

 terval between the first two ribs, and again between the cartilages of the 

 fifth and sixth ribs. After detaching the lining membrane (pleura) from 

 the inner surface of the chest, the student is to cut through the true ribs, 

 except the first and seventh, 1 as far back as he can conveniently reach. 

 The loose sternum and the ribs can be removed by dividing the internal 

 mammary vessels, the triangularis sterni, and the intercostal muscles in 

 the first and sixth spaces. The bag of the pleura, and the cavity with its 

 contents will be now ready for examination. 



The sternum and the cartilages of the ribs will be required hereafter for 

 the dissection of the ligaments. 



Form. The included cavity is irregularly conical, with the apex above 

 and the base down ; and it appears, from the collapsed state of the lungs, 

 to be only partly filled by the contained viscera, but during life the whole 

 of the now vacant space is occupied by the expanded lungs. On a hori- 

 zontal section its shape would appear somewhat cordiform ; for the cavity 

 is flattened on the sides, is diminished in the middle line by the prominent 

 spinal column, and is projected backwards on each side of the spine. 



Boundaries. On the sides are the ribs with their intercostal muscles ; 

 whilst in front is the sternum ; and behind is the spine. 



The base is constructed at the circumference by the last dorsal vertebra 

 behind, by the end of the sternum before, and by the ribs on the side ; 

 whilst the space included by the bones is closed by the diaphragm. 



The base is wider transversely than from front to back, and is convex 

 towards the chest ; though at certain spots it projects more than at others. 



1 The student must be mindful to leave those ribs uncut ; the division of them 

 will not be advantageous to him, and will injure the dissection of the neck and 

 abdomen. 

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