9 CENTRAL NERVOUS SYSTEM. 



observed in different animals. The following considerations may con- 

 tribute something towards a clearer understanding of the true relation 

 which here exists between mass and function. 



A careful examination of Plates XXVI. & XXVII. will convince all 

 that, in the frog, the disparity between the thickness of the brachial and 

 that of the crural enlargements, is due, almost entirely, to a greater 

 amount of white substance in the former region. 



From the plates of Gerlach, made with photographic accuracy, it 

 appears that in man, a non-caudate vertebrate, the principal quantitative 

 variation relates to the gray matter, which, in cross sections, is much 

 more abundant in the lumbar than in the cer\'ical enlargement ; while 

 the white matter seems to be somewhat greater in amount in the latter 

 enlargement, although relatively less than in the frog. 



In the frog, the length of the lumbar stands to that of the cenncal 

 enlargement, in the ratio of lo — 6. In man, they are more nearh' 

 equal in length. The gray matter of the frog's lumbar enlargement, 

 therefore, is more abundant than that of its cer\'ical enlargement, and 

 corresponds, in this respect, with the spinal cord of man ; making up in 

 length for lack of thickness. 



In saurians with long and powerful tails, as the alligator and iguana, 

 the amounts of gray and white substances are nearly the same for both 

 enlargements ; while in those with short and feeble tails, as the horned 

 toad, heloderma and skink, the cervical enlargement, which is of near- 

 ly the same length as the lumbar enlargement, predominates in thick 

 ness, and in the actual amount of both white and gray substances. 



In chelonians, the variations are still more striking and instnjctive. 

 Plates XVI. — XXV. give, from five different species, abundant illustra- 

 tration of the well known fact, that the gray substance of the dorsal re- 

 gion is much more reduced in quantity, than it is in any other verte- 



