RESPIRATION 103 



It roughly corresponds to the borders of the false ribs on both 

 sides. In the right of its centre it is penetrated by the posterior 

 vena cava ; to the left and above its centre it is perforated to 

 receive the oesophagus. Hung on to it are the liver wholly, and 

 the stomach partly, and if it were not for two powerful tendons 

 and muscles let into it from above, known as the crurae, these 

 weights could not be supported. Between the branches of the 

 crurae the posterior aorta escapes from the chest. The tendon 

 of the diaphragm is centrally placed, and forms a dense feltwork 

 of fibres, while the circumferential portion of the organ is mus- 

 cular. The diaphragm is convex towards the thorax, resembling 

 an open umbrella. This convexity is due to the pull exerted on 

 it by the diminished pressure in the air-tight thorax, supple- 

 mented by the pressure of the abdominal viscera from behind. 

 The diaphragm maintains its dome shape towards the thorax so 

 long as the chest is air-tight. Even under pathological condi- 

 tions, when the muscle is burdened by the effusion of gallons 

 of fluid poured into the chest, no flattening of the diaphragm 

 occurs. 



The cavity of the chest is not cut off cleanly and sharply by 

 the diaphragm. The central part of the latter is thrust forward 

 into the chest, and by its projection it separates the two lungs 

 at their posterior part, which in consequence rest on or wrap 

 around the diaphragm, and as it were envelop it. The lungs, 

 when fully distended, do not reach within 8 cm. (3 inches) of 

 the cartilages of the false ribs. These points may be seen in 

 Fig. 44, which gives a side view of the horse's chest. AFE 

 indicates the margin around which the diaphragm is attached ; 

 the dotted line suggests the central part of the diaphragm thrust 

 forward into the cavity of the chest on either side. This is 

 covered by the lungs to within a short distance of the false ribs. 

 If, therefore, the chest were punctured transversely anywhere 

 below the dotted line in the diagram, and the instrument pressed 

 through to the opposite side, the structure would suffer in the 

 following order : lung, diaphragm, abdominal cavity, diaphragm, 

 lung. To state these facts briefly, the largest respiratory area 

 of the lungs lies on an obliquely placed dome-shaped table formed 

 by the diaphragm, which projects into the chest. 



The diaphragm recedes on inspiration, being pushed back into 

 the abdomen, but the retreat does not occur evenly over its 

 whole area. The central part of the diaphragm is naturally 

 restrained by the posterior vena cava, the upper part and ribs 

 move freely, while in the lower half — viz., from the vena cava to 

 the sternum — the movement is again somewhat restricted. As 

 the diaphragm recedes it compresses and carries back the whole 

 of the abdominal viscera, more especially the liver, stomach, and 



