The Hunterian lectures on the evolution of the pelvic floor in the non-mammalian vertebrates and pronograde mammals : delivered before the Royal College of Surgeons of England on March 4th and 7th, 1910 / by R.H. Paramore.

  • Paramore, Richard Horace, 1876-1965.
Date:
1910
Catalogue details

Licence: In copyright

Credit: The Hunterian lectures on the evolution of the pelvic floor in the non-mammalian vertebrates and pronograde mammals : delivered before the Royal College of Surgeons of England on March 4th and 7th, 1910 / by R.H. Paramore. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    visceral pressure in the abdominal r^;ion were increased during expiration, increase in the thoracic pressure would as easily cause a displacement of the lungs posteriorly into the abdomen as it would cause the ejection of air anteriorly, and thus be only partially effective. Indeed, as we ascend the vertebrate series we find the lungs, which in the Dipnoi and Amphibians extend throughout the ccelomic cavity, are bodily compressed more and more anteriorly, until finally they become entirely thoracic organs. This change in position, presumably, is to be attributed to the aspiration of the thorax during the reptilian inspiration and to the increasing activity of the abdo- minal wall during expiration, which, on account of the greater quantity of air inspired, is necessarily amplified in Reptiles, and is therefore of greater intensity than in the terrestrial amphibians— that is to say, greater increments of internal pressure occur during expiration. Similar increase of the internal pressure also occurs during all active move- ments ; for the more intense these become, the more necessary is it for the body wall muscles to contract and give the trunk a certain rigidity ; and even during rest they are maintained in tone, and probably play an important part in keeping up a certain amount of pressure upon the splanchnic venous system, and thus serve as an accessory circulatory mechanism. In accordance with this need for an increased internal pressure, varying from time to time with the require- ments of the moment and the movement of the animal, we find not only a well-developed abdominal wall and a pelvis structurally much stronger and more firmly attached to the sacrum than in the amphibia, but also a more efficient closure of the pelvic outlet assured by the conspicuous lateral caudo-pelvic muscles, which compress the post-pelvic part of the body cavity and the cloacal passage from side to side. The antero-posterior caudo-pelvic muscles in Iguana clearly serve as a fixatory apparatus for the pelvis and thus form the structural basis for the effectiveness of the con- traction of the body wall muscles, a fact which demonstrates the necessity for a tail rigid in the dorso-ventral plane, at least in its proximal part. These antero-posterior caudo- pelvic muscles can scarcely exert much or any pressure upon the cloaca, except perhaps when this is distended by the passage of visceral contents, for their insertion into the pelvis (in Iguana) is lateral and not median. The cloaca, however, is well guarded by the two bow- or crescent-shaped muscles, disposed transversely, the one in front, the other behind ; these together form an efficient sphincter for its orifice, which opens on the surface as a transverse slit. The condition in the Crocodile is essentially similar to that in Iguana. The caudo-pelvic muscles are present, but do not form such fieshy bellies as in Iguana ; they appear rather as parts of the superficial tail musculature, the ventral