334 PHILOSOPHICAL TRANSACTIONS. [aNNO 1787. 



being so flattened in some between the anterior and posterior surface, the passage 

 through these parts is very small or contracted; but the trachea swells out again 

 into a very considerable size. Its larger branches are in proportion to the trunk, 

 and enter the lungs at the upper end along with the blood-vessels. 



Of the lungs. — The lungs are 2 oblong bodies, one on each side of the chest, 

 and are not divided into smaller lobes, as in the human subject. They are of 

 considerable length, but not so deep between the fore and back part, as in the 

 quadruped, from the heart being broad, flat, and of itself filling up the fore 

 part of the chest. They pass farther down on the back part than in the quad- 

 ruped, by- which their size is increased, and rise higher up in the chest than the 

 entrance of the vessels, coming to a point at the upper end. From the entrance 

 of the vessels they are connected downwards, along their whole inner edge, by 

 a strong attachment (in which there are in some lymphatic glands) to the posterior 

 mediastinum. The lungs are extremely elastic in their substance, even so much 

 so as to squeeze out any air that may be thrown into them, and to become almost 

 at once a solid mass, having a good deal the appearance, consistence, and feel of 

 an ox's spleen. The branches of the bronchiae. which ramify into the lungs have 

 not the cartilages flat, but rather rounded; a construction which admits of 

 greater motion between each. The pulmonary cells are smaller than in quad- 

 rupeds, which may make less air necessary, and they communicate with each 

 other, which those of the quadruped do not ; for by blowing into one branch of 

 the trachea, not only the part to which it immediately goes, but the whole lungs 

 are filled. 



As the ribs in this tribe do not completely make the cavity of the thorax, the 

 diaphragm has not the same attachments as in the quadruped, but is connected 

 forwards to the abdominal muscles, which are very strong, being a mixture of 

 muscular and tendinous fibres. The position of the diaphragm is less transverse 

 than in the quadruped, passing more obliquely backwards, and coming very low 

 on the spine, and higher up before; which makes the chest longest in the direc- 

 tion of the animal at the back, and gives room for the lungs to be continued 

 along the spine. 



The parts immediately concerned in inspiration are extremely strong; the 

 diaphragm remarkably so. Thie reason of this must at once appear ; it necessa- 

 rily requiring great force to expand in a dense medium like water, especially too 

 when the vacuity is to be filled with one which is rarer, and is to water a species 

 of vacuum, the pressure being much greater on the external surface than the 

 counter-pressure from within. But expiration on the other hand must be much 

 more easily performed; the natural elasticity of the parts themselves, with the 

 pressure of the water on the external surface of the body, being greater than 

 the resistance of the air from within, will both tend to produce expiration with- 



