124 Mr. Le Gros Clark on the Mechanism of Respiration, 



perty, in association with the attachments of the diaphragm, and the 

 relations of other muscles credited severally with functions in both inspi- 

 ration and expiration. 



8. The thoracic parietes, both osseous and cartilaginous, are movable, 

 admitting of an increase or diminution in the circumference of the chest ; 

 and the diaphragm is attached, by its circumference, to the chest-walls. 

 As the force by which the tensely arched diaphragm is rendered plane is 

 necessarily considerable, it may be asked, does the contraction of this 

 muscle under any circumstances draw in the ribs ? and why does it not 

 do so always ? I believe the answer to the first inquiry to be, that it 

 does, under some circumstances, draw in the ribs ; and the explanation of 

 the second must be sought in a consideration of the agency of other 

 muscles engaged in inspiration, to which attention will be presently 

 directed. 



9. The property of the diaphragm under consideration — its passive ten- 

 sion — is engaged in restoring the equilibrium of rest after a deep expira- 

 tion. The supplemental air is expelled from the lungs chiefly by the 

 action of the abdominal muscles, the circumference of the chest undergoing 

 no appreciable change except at its lower part, where it is directly acted 

 on by these muscles. "When they are relaxed, and the lower costal region 

 again expands, the diaphragm asserts its passive influence, by drawing in so 

 much air as the counter-resistance of the contractility of the lung-tissue 

 will permit. 



10. But this agency is somewhat more complex than would appear 

 from the above statement. The passive tension of the diaphragm is such 

 that it would probably be sufficient to resist the efforts of the abdominal 

 muscles to force it upwards, without the cooperation of another cause, 

 which is the altered condition of the chest-walls. The lower ribs are per- 

 ceptibly drawn inwards, as stated, when the supplemental air is expelled 

 from the lungs; and, as a necessary consequence, the horizontal portion of 

 the diaphragm is relaxed, and thus placed in a condition which permits of 

 its being forced upwards, so as to compress the lungs. 



11. But the deepest inspiration is also attended by hollowing or draw- 

 ing in of the epigastrium ; and the increasing circumference of the lowest 

 costal region, which is limited as compared with the upper, [is almost 

 arrested before the act of forced inspiration has attained its maximum ; 

 and the rise in the abdomen is likewise suspended before this climax is 

 reached. These conditions are due, I apprehend, to the combined effect 

 of atmospheric pressure on the relaxed abdominal walls, concurrently with 

 the extreme contraction of the diaphragm, overcoming the resistance of 

 the intercostal muscles, and drawing in the lower ribs. 



12. As exemplifying, in a remarkable and interesting way, the fore- 

 going observations, I may mention the case of a patient recently an in- 

 mate of St. Thomas's Hospital, in whom there was fracture, with dis- 

 placement of the sixth cervical vertebra, by which the cord was compressed. 



