ANATOMY. 



ORGANS OP HKSPIHATION. 



As these are contained in the caVity of 

 *he thorax, we shall consider the subject 

 in the form of a description of that cavity 

 and its contents. 



The cavity of the thorax is the space 

 included by the dorsal vertebrae behind, 

 by the ribs with their cartilages, the ster- 

 num, and intercostal muscles, at the sides 

 and fore part ; and by the diaphragm be- 

 low. This cavity is lined by a membrane 

 called the pleura, which has a smooth in- 

 ternal surface, constantly moistened by a 

 serous exhalation. 



The cavity of the chest contains two 

 distinct membranous bag's, called th.e 

 right and left bags of the pleura ; each 

 of these holds the lung of its own side, 

 and is entirely separated from the oppo- 

 site one. The pleura not only forms a 

 bag which holds the lung, but is also re- 

 flected over the surface of the viscus, be- 

 stowing on it a smooth exterior invest- 

 ment. This is called the pleura pulmo- 

 nalis, to distinguish it froin the other, 

 which is named pleura costalis. 



If the cartilages of the ribs be divided 

 on one side of the chest, the correspond- 

 ing bag of the pleura will be opened; and 

 it will then appear, that this is separated 

 from the opposite one by a partition, which 

 extends from the sternum in front to the 

 vertebrae behind, and is known by the 

 name of mediastinum. The pleura may 

 be compared to two bladders placed late- 

 rally with respect to each other, but adher- 

 ing only partially, and separated by vari- 

 ous intervening bodies. Thus, the heart 

 and adjoining large blood-vessels, the 

 oesophagus, and the division of the trachea 

 into the two bronchi, are placed between 

 the two pleurae. The mediastinum then 

 is the space included betweenthe opposed 

 surfaces of the two bags of the pleurae, 

 and containing the parts above mentioned. 

 The name of anterior mediastinum is ap- 

 plied to a small interval left between the 

 two pleurae, just behind the sternum, and 

 occupied only by a loose cellular texture. 

 The posterior mediastinum is a larger 

 space in front of the bodies of the verte- 

 brae ; it contains the descending aorta, the 

 vena azygos, thoracic duct, oesophagus, 

 and the par vagum. 



The capacity of the chest taken altoge- 

 ther varies, according as we estimate it in 

 a state of inspiration or expiration ; being 

 largest in the former, and smallest in the 

 latter state. The right bag of the pleura 

 is considerably larger than the left, as is 

 also the right lung. 



In the living state, the lung is in close 

 contact with the surface of the cavity, and 

 follows all the motions of the sides of the 

 chest. It is distended by the influx of air, 

 when the chest is enlarged ; and the air is 

 expelled from the lung, when the chest is 

 diminished. As soon as the thorax is 

 opened in the dead subject, the lung falls 

 down from the sides of the chest, or, in 

 technical language, collapses, and then a 

 large empty space is seen between it and 

 the ribs. From this representation it 

 should appear, that the lungs are quite 

 passive in the business of respiration. 



The lungs are two in number : one be- 

 ing contained in each bag of the pleura. 

 They are loose and unconnected in these 

 bags, except atone point, towards the up- 

 per and posterior portion of each vi.scus, 

 where the great vessels enter them, and 

 where the bag of the pleura is continu- 

 ous with the reflected portion of thfrihem- 

 brane. These are called the ligaments of 

 the lungs. 



Their colour varies considerably. It is 

 always verging more to a red, in propor- 

 tion as the subject is younger: in the adult, 

 it has more of a spotted and livid cast. 

 Towards the back of the lungs it is always 

 much deeper, from the gravitation of blood 

 in the vessels in consequence of the posi- 

 tion of the subject. It is lighter, when the 

 lungs contain much air. 



The lungs are subdivided into lobes ; 

 of which the right contains three, and the 

 left two. Their substance is composed of 

 a congeries of minute membranous cells, 

 about equal in size to a pin's head, and as 

 these are more or less filled with air, they 

 give the lung a peculiar spongy feel. 

 These cells communicate with the ulti- 

 mate ramifications of the air vessels, and 

 receive air from that source. The pul- 

 monary vessels ramify minutely in them, 

 and thereby expose the blood to the ef- 

 fects of the contained air ; and in this ex- 

 posure the object of respirationis effected. 



T/ie -windpipe. The tube, which con- 

 veys the external air into the lungs, may 

 be divided into three parts ; the larynx, 

 the trachea, and the bronchi. 



The larynx is a hollow cartilaginous or- 

 gan, placed at the top of the trachea. The 

 air which passes through this from the 

 lungs, in expiration, produces the voice. 



The cavity of the larynx opens above at 

 the root of the tongue, and below into the 

 trachea. The organ is composed of live 

 pieces of cartilage viz. the thyroid and 

 cricoid cartilages and epiglottis, and two 

 arytenoid cartilages. 



The thyroid cartilage is the largest, and 



