520 THE LUNGS. 



these can be distinctly opened. The contractibility of the pul- 

 monary tissue under the influence of galvanism observed by 

 Dr. C. J. B. Williams, seems to establish the muscularity of the 

 circular fibres. 



On the posterior membranous portion of the trachea, where 

 the cartilages are deficient, a considerable number of small 

 glandular bodies are placed, which are supposed to communicate 

 with the mucous ducts that open on the internal surface. If 

 these bodies are removed from the external surface of this 

 portion, and the muscular fibres are also removed from the 

 internal, a very thin membrane only remains, which is very 

 different from that which is left between the rings, when the 

 fleshy substance is removed from that situation. 



The reason of the deficiency in the rings, at this posterior 

 part, is not very obvious.* It continues in the bronchia until 

 the form of their cartilages is changed in the lungs : if it were 

 only to accommodate the oesophagus, during the passage of 

 food, there would be no occasion for its extension to the 

 bronchia. 



At the bifurcation of the trachea, and on the bronchia, are a 

 number of black colored bodies, which resemble the lymphatic 

 glands in form and texture. They continue on the ramifications 

 of the bronchia some distance into the substance of the lungs. 

 Their number is often very considerable ; and they vary in size 

 from three or four lines in diameter to eighteen or twenty. As 

 lymphatic vessels have been traced to and from them during 

 their course to the thoracic duct, they are considered as lym- 

 phatic glands. 



Of the Lungs. 



There are two of these organs : each of which occupies one 

 of the great cavities of the thorax. 



When placed together, in their natural position, they resem- 

 ble the hoof of the ox, with its back part forward ; but they are 



* Dr. Physick has advanced the opinion that it enables a person to expel the 

 mucus of the lungs by contracting the size of the trachea, and consequently 

 increasing the velocity or impetus of the air. H. 



