1. Six or seven years since, while examining the chest in persons subject to dyspnœa, I was struck by observing that the latissimus dorsi and the serratus magnus muscles acted during forcible expiration; on further inquiry, I found that neither of them acted during inspiration. I also ascertained that, on inspiration , the scaleni acted during the whole time; that the superior ribs came nearer to each other, and the inferior moved farther apart; and that the internal intercostal muscles between the six superior costal cartilages and the external intercostal muscles between the superior ribs were in action. I communicated these observations to Professor Bell; he advised me to examine and figure the muscles of respiration in the lower animals. This I did.