15^ 



VESTIGES OF THE 



is the only representation of a spinal marrow. Now in 

 this state it perfectly resembles the brain of an adult fish, 

 thus assuming hi transitu the form that in the tish is 

 permanent. In a short time, however, the structure is 

 become more complex, the pai-ts more distinct, the spinal 

 marrow better marked ; it is now the brain of a reptile. 

 The change continues ; by a singular motion, certain 

 parts {cor2)ora quadrigemina) which had hitherto ap- 

 peared on the upper surface, now pass towards the 

 lower ; the former is their permanent situation in fishes 

 and reptiles, the latter in birds and mammalia. This is 

 another advance in the scale, but more remains yet to be 

 done. The complication of the organ increases ; cavities 

 termed ventricles are formed, which do not exist in fishes, 

 reptiles, or birds ; curiously organised parts, such as the 

 corpora striata, are added ; it is now the brain of the 

 mammalia. Its last and final change alone seems 

 wanting, that which shall render it the brain of man." * 

 And this change in time takes place. 



So also with the heart. This organ, in the mammalia, 

 consists of four cavities, but in the reptiles of only three, 

 and in fishes of two only, while in the articulated 

 animals it is merely a prolonged tube. Now in the 

 mammal foetus, at a certain early stage, the organ has 

 the form of a prolonged tube ; and a human being may 

 be said to have then the heart of an insect. Subsequently 

 it is shortened and widened, and becomes divided by a 

 contraction into two parts, a ventricle and an auricle ; it 

 is now the heart of a fish. A subdivision of the auricle 

 afterwards makes a triple-chambered form, as in the 



* Lord's "Popular Physiology." It is to Tietk'iiiannlliat avc chietly 

 owe these curious observations ; but ground was lirst broken in this 

 branch of physiological science by John Hunter. 



