18 ON THE NERVOUS SYSTEM IV. 



SECTION III. SPINAL CHORD. 



As is almost universally the case in the Vertebrate series, this portion of the nervous 

 system consists of a flattened cylinder terminating posteriorly in a conical extremity, 

 and is partially divided lengthwise by two fissures, one on the upper and the other 

 on the under surface ; the latter in Frogs is the most easily traced, and when the 

 membranes are stripped off, the right and left halves readily separate as far as the 

 gray substance. By a glance at Plate I. Fig. 1, it will be at once seen, that there 

 exist in the chord three enlargements or bulgings, I, L, N, at nearly equal distances. 

 The first of these, the medulla oblongata, is almost universally present in Vertebrate 

 animals, being perhaps really deficient in Amphioxus only; but the second and 

 third are wanting whenever the arms and legs are not developed, or exist in a 

 rudimentary condition. Among Sauria the genus Bipes possesses only the third 

 or crural, and Chirotes only the second or brachial enlargement ; legs only being 

 developed in the first, and arms in the second. 



When seen from the abdominal or lower side, as in Fig. 1, these three enlarge- 

 ments have a strong resemblance to each other, and it is at once suggested, that 

 they are serial repetitions of similarly constructed parts. In addition to these, I 

 have noticed in a few instances a slight enlargement of the chord at the origin of 

 each of the pairs of spinal nerves, and from observations made on other animals 

 am inclined to the belief, that a similar condition of the chord exists more 

 generally than anatomists have been disposed to admit, and that there is con- 

 sequently more truth in the disputed statements of Gall and Spurzheim with 

 regard to this point of anatomy than they have generally received credit for. 

 The enlargements in question are quite obvious in Menobranchus ; also in 

 Lophius Americanus, as stated by Cuvier, though the correctness of this last 

 statement has been called in question by Professor Owen. It is possible, how- 

 ever, that in the European species it may not exist; in the American, it is un- 

 equivocally present. 



From the second or brachial enlargement is given off a large trunk distributed 

 to the arms, and beyond this a smaller one, which sends a filament to the preceding 

 nerve ; the contracted portion of the chord which follows the brachial enlargement 

 gives off the nerves to the walls of the abdomen, and the third enlargement sup- 

 plies the three lumbar nerves and the minute coccygeal pair. The second and 

 third bulgings present no obvious difference as regards structure, but the principal 

 interest which attaches to them is the proportion between their size and the limbs 

 with which they are connected. It is stated by Cuvier, that they are in pro- 

 portion to the " force " of their respective limbs. If by force is meant the 

 muscular energy and development of the limbs, this statement does not appear 

 to be sustained in the present instance, nor in many other instances brought 

 to notice by comparative anatomy. In man the brachial enlargement is always 

 larger than the crural, though the legs are so much more powerfully developed 

 than the arms, and the same is true of the greater number of Mammals. 

 In Frogs there is a still greater disproportion between legs and arms, yet 



