IN THE LIMBS OF MAMMALIA. 17 



such marked preponderance of the anterior portion as in the later periods, but the two 

 ends present a nearly similar appearance, which similarity is persistent in the lowest fishes. 

 There seems to be no good reason for regarding the anterior enlargement of the myelon 

 as essentially or morphologically distinct from the posterior. The brain is an after-growth, 

 for teleological cause, and the posterior enlargement is distinct, and, though entirely over- 

 balanced by the immense development of the organs of the mind and special senses, is of 

 no small importance even physiologically. No one who has suffered the excruciating pain 

 in the small of the back, accompanying most febrile diseases, will question the importance of 

 the portion of the myelon there situated, to which part also are referred the sensations 

 of relief, more or less distinctly felt, upon the discharge of the contents of intestine, blad- 

 der, uterus or testis. 



To avoid misconception, it may here be stated that from this morphological point of 

 view, I consider the brain proper as a purely physiological addition with no posterior repre- 

 sentative, and the so-called anterior and posterior enlargements of the spinal cord as bun- 

 dles of the fibrous, that is, connecting or adynamic nervous substance, increased in size 

 and number ; but the medulla-oblongata as the true anterior morphological enlargement, 

 the posterior representative or longitype of which is that apparently insignificant portion of 

 the cord which lies behind the origins of the great lumbar nerves, but which contains a 

 very large proportion of the gray or cellular or dynamic nervous matter. 



But this point, and many others, demand investigations more rigid, minute, and even 

 microscopic, and with reference to the principle involved. The idea of antero-posterior 

 symmetry is as yet illustrated to us only by such parts and organs as conform to it mpre 

 obviously by teleological likeness or unlikeness in the two regions of the body ; and, though 

 there is already enough to establish beyond a doubt, to my mind at least, the truth of this 

 subdivision of the higher law of polarity, yet there is needed more time and labor than 

 any one has yet been able to bestow, to demonstrate the more obscure relations of those 

 parts which are only morphologically anterior and posterior repetitions of each other. 



It is by no means unlikely that further investigations, broad and impartial, may show 

 that, except between certain organs, the relation of polar homology is a general and not a 

 special one ; at any rate, I think we are not yet prepared to state what bones of the head 

 and pelvis are longitypes ; if mere consolidation and organic connection of the more an- 

 terior vertebrae constitutes the cranium, then many fishes have five or six, or even more 

 cranial vertebrae, while in the frog the occipital segment is so divided that the two lateral 

 elements, or neurapophyses, are separated from the centrum and neural spine, and so appear to 

 constitute a distinct vertebra. 



Perhaps the two or even the three anterior cranial vertebrae, the nasal, frontal, and 

 parietal, are, like the special senses, not represented posteriorly. It may be said in 

 favor of this view, that, while the universal sense of touch is perfected in 'the hands, which 

 are the distal ends of the diverging appendages of the lower or haemal arch of the poste- 

 rior or occipital cranial vertebra, and taste, the modification of this universal sense, is 

 located in the tongue which is supported by the haemal arch of the next or parietal verte- 

 bra, the organs of the three special senses are located in or between the superior or neural 

 arches of this and the two remaining segments, which indeed seem to exist only with ref- 

 erence to these and to the brain proper, none of which have posterior representatives ; 

 therefore, we might not expect to find in the pelvis any parts corresponding exactly to 

 these special developments, but only the entire vertebra whose haemal arch has for its 

 diverging appendages the posterior extremities, with that portion, hasmapophysial, of the 



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