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Show 336 MR. W. WOODLAND ON THE [Apr. 21, and enquiry confirms the supposition. Though, in the above account of mammalian locomotion, most stress has been laid upon the impulsive elevations of the body and resulting strains on the attachments of organs, yet it must not on that account be inferred that 011 descent of either half of the trunk the shock consequent on contact of the limbs with the earth (equal in intensity to the elevatory impulse) is negligible. (See above in case of Kangaroo in which it is taken into account.) Such shocks have the effect of causing those viscera which are closely adherent to the mass of the body either to exert a considerable pressure on structures anterior to them or to be dislodged from their normal position; and if in either case such a result is to be avoided, organs of support must be developed. In mammals, the lungs together with the heart occupy the anterior portion of the coelomic cavity, and behind these are situated the liver, stomach, and intestines, these together constituting a considerable mass. These massive organs, unless prevented, would on each contact of the limbs with the earth exert great pressure on the fragile compressible lungs immediately anterior to them. Hence, in order to obviate ensuing derangements, we find in all mammals, and in many other terrestrial animals, a stout partition separating the cavity of the liver and gut from the cavity of the lungs-a structure which can only have been originated by natural selection. Evidence supporting the conclusion that the diaphragm arose as an adaptation to the forward pressure of the liver and gastric mass, is not only to be found in the fact that the diaphragm is convex anteriorly, but also in that the convexity is, cceteris paribus, proportional to the impulsiveness of the animal's activity and to the mass of the liver and gut. In herbivores the mass of the gut is greater than in carnivores; on the other hand, " carnivorous (fat-eating) animals generally possess a larger liver than herbivores" (Wiedersheim), so that these respective characters tend to defeat comparison \ But in the Perissodactyla we find a fairly large liver (considerably larger than in Artiodactyla), a long herbivoran gut, and a high degree of impulsiveness, the combination of which on our hypothesis should be correlated with a highly convex diaphragm__an inference which proves correct. Owen remarks that " in the perissodactyle Ungulates, in which the movable ribs are numerous and continued to near the pelvis, the diaphragm is also extensive and much arched towards the thorax "-indicating that the convexity of the diaphragm is markedly above the normal. And, doubtless, other illustrations are adducible. Additional evidence as to the primary function of the diaphragm is, moreover, afforded by the case of the Struthiones (and allies) which have adopted the mammalian mode of locomotion. In these birds there exists a " well-marked diaphragm forming a partition which divides the thoracic cavity into two parts, one posterior and small containing the lungs, and the other anterior and large containing the heart 1 The small capacity of the herbivoran thoracic cavity as compared with that of the carnivora must be remembered in this connection. |
