IN THE LIMBS OF MAMMALIA. 9 



structure, however, the fore-leg gives way suddenly and completely when the animal 

 stumbles so as to bear upon the tip of the hoof and so flex the hand at the wrist, for that 

 brings the humerus down with it. In the hind-leg of the frog, which is used for little else 

 than leaping, there is a somewhat similar arrangement, the great extensor of the foot being 

 connected with that of the leg by a strong tendinous band on the inner side of the knee, 

 so that extension of one segment is mechanically connected with that of the other. In the 

 quadrumana and carnivora the condyles are present, though less prominent than in man ; 

 and indeed the degree of their development seems to correspond nearly with that of the 

 clavicle, both of them being concerned in the freedom and mobility of the anterior ex- 

 tremity. 



The new relations of morphology observed among the muscles of the mammalian limbs 

 are intimately connected with two other generalizations applying to these parts ; and these 

 again are subordinate to the great anatomical law of " antero-posterior symmetry," as it 

 has been hitherto called ; and since little or nothing has been published concerning this in 

 the form it has of late assumed, it may be well to state the law here, chiefly according to the 

 views of Professor Jeffries Wyman, by whom it was suggested to me, and who, almost alone 

 in this country, has devoted time to eliminating, from the indefinite and often extravagant 

 and absurd shape in which it was left by Oken, the real truth of a principle the most potent 

 and elevated of which the vertebrate body, considered by itself, is capable. 



Yet in my opinion even this is subordinate to a still higher law which pertains as well 

 to the other types of the animal kingdom, and also coincides with a geometrical law so 

 closely as to afford new ground for its belief. In order to appreciate the full force and 

 value of lesser laws, it must first be shown how they depend upon greater ones ; and there- 

 fore the latter shall be first considered. 



A partial statement of this higher law, which for reasons given further on I have called 

 the law of animal polarity, was made by Professor Agassiz, at a meeting of the Boston Soci- 

 ety of Natural History, December 4th, 1861. 



He characterized the four leading types of the animal kingdom by four terms indicative 

 of the general arrangement of their organs, or their plan of structure : the Radiates by 

 " radiality" the Mollusks by " laterality" the Articulates by " tergality" and the Vertebrates 

 by " cephality" In the Radiates all the parts are disposed about a common centre, encircling 

 which also is the dynamic portion of the nervous system, a ganglion for each diverging 

 segment or spheromere. These spheromeres are morphologically exact repetitions of each 

 other, though their size and shape may be greatly modified, and even one of them may be 

 entirely wanting, so that the animal appears as if divisible into two lateral halves, when 

 really this is due to a teleological modification not at all affecting the real plan of structure, 

 but only foreshadowing, as it were, the characteristic arrangement of the next higher type, 

 just as the molluscan Bryozoa present an appearance of radiation in the disposition of 

 their groups of tentacles. These two instances show the importance of always looking first 

 at the more essential parts of the body, rather than at the outside, which, like other ap- 

 pearances, is often deceitful. 



The laterality which Professor Agassiz considers characteristic of the Mollusks, must 

 be carefully distinguished from the bilaterality or bilateral symmetry of all animals above 

 Radiates : for the latter terms mean only that the body is composed of two lateral halves 

 which are right and left repetitions of each other ; and this is often more conspicuous in 



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