It E S P 1 It A T1D \ I N T II E C H E L ONIA. 5 



and the intestinal canal, liver, &c, removed, the lungs, consisting of two lobes, an; 

 seen covering nearlj the whole of the testae; (hoy are cellular, as in the frog, and 



consist of two lobes, one on each side of the spina dorsi, each of which is sub- 

 divided into four or five indistinct lobules. The cellular texture of these is not 

 uniform; the cells of the middle lobules being tin- smallest, and those of the last 

 lobule the greatest; this lobule is likewise- loose, not being tied down on the sides 

 nor beneath, the rest are tied down to the spine. My attention was soon called to 

 observe the structure and office of some muscles in the region of the flanks, which 

 I observed often to be in motion, contracting and extending alternately, and though 

 placed by the side of the hind legs, these were not moved by them. Further, the\ 

 were placed at the end of the last lobule of the lungs, and they appeared to retain 

 their irritability the longest. This was sufficient to lead me to conjecture that 

 these might be the parts by which respiration in these animals was performed; and 

 to see them act in their natural position I sawed off, in another tortoise, that part 

 of the shell which covers them, and I then saw them constantly working. One was 

 now placed nearly in a perpendicular direction, and another, or part of the same, 

 was placed nearer the sternum, lying almost in a horizontal direction. The first in 

 its contraction receded from the testa inwards, whilst the latter, in its contraction, 

 observed a contrary direction. When I attributed to them the office of expirator 

 and inspirator muscles, which I supposed them to perform, I was embarrassed, 

 because I could not conceive how a muscle could be a constrictor with its convex 

 side; yet when the expirator, by contracting, had receded from the shell inwards, 

 it appeared, when viewed from without, to be concave. But this difficulty ceased 

 as soon as I had opened the animal and dissected the parts, for I then found the 

 following admirable contrivance of nature. This part is composed of two distinct 

 muscles, with their risings and insertions quite different, yet firmly connected in the 

 middle by cellular membrane. The first rises from the testa near the spina dorsi, 

 and is inserted into the peritonaeum; this is the constrictor of the lungs, or the 

 muscle of expiration. The other is nearly spread over the whole cavity between 

 the upper and under shell, where the hind legs are drawn in during the contracted 

 state of the animal, being inserted into the margin of the testa above, and the mar- 

 gin of the sternum below. The places of insertion of these muscles, and their con- 

 nection in the middle being known, there is then no difficulty in explaining why 

 the muscle, while acting as a constrictor, appeared concave, as it was only the 

 inspirator brought into that position by its antagonist; nor any difficulty in con- 

 ceiving how they carry on the function of respiration; for the expirator being 

 connected, as I have already said, to the testa below and to the peritonaeum above, 

 envelops in a manner the last movable lobule of the lungs; when, therefore, it con- 

 tracts, it compresses this part of the lungs, and by that means expels the air; then 

 ceasing to act, the other contracts, and draws the former with it, thus a vacuum is 

 formed, into which the air rushes, as in the respiration of those animals which have 

 a thorax. 



To prove that this explanation was well-founded, and that the motions of these 

 muscles were really those of respiration, I made the following experiment. I 

 fastened on the nose of a tortoise a little valve made of white paper, which covered 



