26 ANATOMY AND PHYSIOLOGY OF 



following verj simple experiments converted this conviction into the most absolute 

 certaintj — a certainty which every future step served but to illustrate from new 

 points of view. 



On page 11 of the memoir of Dr. Weir Mitchell, previously cited, are to be found 

 the experiments above alluded to. The trachea of a large snapping turtle was cut 

 across, after which breathing went on at the usual rate, or more often, owiug to 

 causes presently to be mentioned. Next, a bent glass tube, two millimetres in 

 width, was adapted to the upper or outer end of the divided trachea, and allowed 

 to dip into water. If the breathing power resided in the hyoid arches, larynx, and 

 mouth, the water in the tube should have been forced downwards during inspiration, 

 but, although respiration continued, the fluid moved at this time., only about one 

 millimetre, and even this was plainly due to the motion of opening and closing the 

 glottic lips, which occurs synchronously with the respiratory movements in the 

 breast-box. 



The same bent tube Avas next adapted securely to the lower end of the divided 

 trachea, and again dipped into water as before. At each subsequent inspiration 

 the water was forcibly and hugely drawn up into the lung, and again rejected during 

 expiration. After this no doubt could exist as to the locality in which arose the 

 mechanical force productive of respiration. With this convincing proof the subject 

 was laid aside for the future and more thorough investigation, of which this cssa\ 

 is the record. 



Function of flu respiratory muscles of th Turtle. — A large snapping turtle- was 

 secured on its back, and an incision made over the flank space, between the pos- 

 terior limb and the plastron and carapace. The skin and superficial fascia were 

 then carefully removed so as to expose the whole muscle which fills this space, and 

 which has already been fully described. 



When inspiration took place, the muscle contracted, and as it is possessed of a 

 central tendon from which radiate fibres in all directions, the result of their 

 shortening was to convert its previous deeply concave surface into one which was 

 nearly level, while at the same time the air rushed through the open glottis into the 

 lung. The analogy between this muscle and the diaphragm of mammals was abso- 

 lutely perfect. The central tendon, the converging muscular fibres, and the form of 

 movement resulting from this beautiful arrangement, all united to suggest the 

 resemblance. The inspiratory function of this muscle was palpably evident, nor 

 could any other office be possibly assigned to it, because it was attached to no 

 movable bouc or other parts susceptible of motion. 



Repeated galvanization of this muscle served further to demonstrate its purpose. 

 Finally, the muscles on both sides were removed, when all inspiratory power was 

 lost. The turtle could empty its lungs, but possessed no power to till them anew. 



The muscles engaged in expiration were next made the subject of study. At 

 first we were led to believe, that the elastic contractility of the lungs might alone 

 suffice to empty them, but this was opposed to all physiological analogy, and the 

 power with which expiration occurred was too great to allow us to suppose that no 

 muscular force intervened for its production. 



To examine this part of the subject, a turtle (snapper) was secured, as usual, and 



