58 



37. The heart is placed in the middle of the lungs, 

 which consist of two parts, the right and left lobe. 

 Each of these is divided into two other lobes ; one of 

 these frequently into three, sometimes into four, by 

 fissures, sometimes deeper, sometimes shallower, run. 

 ning from the interior margin towards the back part. 

 The lungs are divided into cells, -which are no other 

 than expansions of the small branches of the trachea 

 *>r wind pipe. And there is an easy passage from 

 one branch into many cells, and again into it from 

 them all. The upper part of the trachea opens into 

 the mouth ; the lower, divided into two branches, 

 shoots out into various ramifications, which are termed 

 bronchia. And these little canals running on, consti- 

 tute the lungs, whose cells are wonderfully connected 

 together, and intermixed with numberless branches of 

 veins and arteries. 



The upper end of the trachea is called the larynx. 

 At the fourth vertebra of the back, it divides and en. 

 ters the lungs. Its cartilages, ranged at small and 

 equal distances, are smaller and smaller as they ap- 

 proach the lungs. 



These cartilages have two membranes. The exter- 

 nal, composed of circular fibres, covers 'the whole 

 trachea. That which lines it within, consists of three 

 distinct membranes, the first woven of two orders of 

 fibres, part longitudinal, to shorten it by drawing the 

 cartilages together : part circular, to contract them. 

 Both these, \< Aether with the external membrane, 

 assist in breath, ng, coughing, and varying the tone of 

 voice. The second membrane is glaudulous ; and its 

 glands opening into the cavity of the trachea, separate 

 a liquor which moistens and defends it from the acri- 

 mony of the air. The third is a net- work of nerves, 

 veins, and arteries. 



The organs which form the voice of man, have not 

 been accurately observed by the ancients. As the 

 trachea bears some resemblance to a liute, they con- 

 sidercdtiic voice, according to the sounds of that in- 



