Researches upon the anatomy and physiology of respiration in the Chelonia / by S. Weir Mitchell and George R. Morehouse.

  • Mitchell, S. Weir (Silas Weir), 1829-1914.
Date:
[1863]
Catalogue details

Licence: Public Domain Mark

Credit: Researches upon the anatomy and physiology of respiration in the Chelonia / by S. Weir Mitchell and George R. Morehouse. Source: Wellcome Collection.

Provider: This material has been provided by the National Library of Medicine (U.S.), through the Medical Heritage Library. The original may be consulted at the National Library of Medicine (U.S.)

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    which loosens the entire muscular and bony mass, and facilitates its detachment. The section should be made with the lung partially inflated, to secure from injury the anterior belly of the expiratory muscle, which lies in contact with the posterior surface of the serratus magnus. The further removal of the tissues of the flank, and their careful separation from the posterior belly of the expiratory muscle to which they are adherent, completes the exposure. Looking at the result of our dissection, we find a tendinous and muscular sac occupying the dorsal shield, filling its entire width in the middle and most of its length; its general form is cordate, the apex dipping into the pelvis, and its anterior notch giving place to the heart and the muscles and vessels of the neck. Much the larger portion of the sac visible is tendon (Fig. 3, g, g'), and has hitherto been regarded as peritoneum, but a closer scrutiny would have revealed its fibres gather- ing from all sides towards an oval centre, in which they are inseparably interwoven. The tendon in many places can be lifted from the peritoneum, by which it is lined. Curving around its anterior and posterior borders are muscular fringes (Fig. 3, d, d' and c), the fibres running from the carapace in lines parallel to the long axis of the trunk. These are the anterior and posterior bellies of the expiratory muscle, the diaphragmaticus and transversus abdominis of Bojanus. These muscles are inserted into the common tendon, and in contracting compress the contained viscera against the shell and expel the air from the lungs. Dividing the tendon through its middle from side to side, and removing the abdominal organs and permitting the lungs to collapse, we are enabled to obtain a satisfactory view of the origin of these muscular bellies from within. The posterior belly arises from the pelvic fascia from a point opposite the anterior third of the ilium backwards to the spine, from the eighth vertebra, and by tendinous fibres from the carapace as far as the sixth rib, the line of origin slowly leaving the spinal column as it reaches forwards. Turning outwards at an obtuse angle, after joining the sixth rib, the muscle follows its posterior edge until near its extremity, where it inclines forwards and terminates at the fifth rib as it joins the marginal plates, a point corresponding very nearly with the pelvic end of the suture connecting the carapace and plastron. From this sigma-shaped origin the fibres curve backwards and downwards, embrac- ing the abdominal viscera, and unite with the tendon below, forming a regular and well-defined line, varying in position as the muscle is contracting or at rest. Fig. 3, c, represents the lungs distended and the muscle relaxed. This belly, considered by itself, is a strong membranous muscle, somewhat triangular in shape, the apex being at the edge of the shell, and the base at the pelvis. In a turtle weighing sixteen pounds, the fibres at the apex measured one-half inch in length, while in the middle and at the base they measured respectively five and one-half and four inches. The anterior belly arises from the vertebral margins of the second and third intercostal spaces, from the second costal arch, from the second rib along two-thirds of its length, and across the carapace in a line curving backwards and outwards, from the third and fourth ribs, near their junction with the marginal plates. It will thus be seen that the outermost origins of the anterior and posterior bellies closely approxi-