IV. OF RANA riPIENS. 19 



there is not a corresponding difference in the size of the bulgings. They cannot, 

 therefore, be said to be in proportion to the muscular force only of the limbs, 

 but correspond far more nearly to the acuteness of the sense of touch, which in 

 Man and Mammals is more delicate in the hands and arms than in the legs and feet. 

 In Bats, it is true that the muscular force of the arms is greater than that of the 

 legs, and that the brachial far surpasses the crural enlargement ; but, at the same 

 time, the sense of touch in the membranes of the wings is exalted to a most extra- 

 ordinary degree. In Birds the posterior bulging is almost universally the largest, 

 though this condition is in part dependent upon the presence of the rhomboidal 

 sinus. In these animals, while the muscular energy of the wings is the most de- 

 veloped, the sensibility of the feet is the more acute. 



A transverse section of the chord, when viewed with a low magnifying power, 

 exhibits the external or white substance forming two crescentic masses, as in 

 Fig. 1, a, their concavities turned towards each other and ^'s- '• 



including the gray substance, h ; this last seems to form 

 the only bond of union between the two sides, (and in 

 this respect agrees with the results of Mr. J. L. Clarke,* 

 from an examination of the chord in Man,) the white col- 

 umns being quite distinct from each other, and having 

 no commissural fibres either on the anterior or the pos- 

 terior face. Under the microscope, the white substance 

 is resolved wholly into nerve tubes, and in the gray substance even, these are the 

 most conspicuous element. The largest portion of the white columns consists of 

 longitudinal nerve tubes, intermixed with others which are transverse; some of 

 these last are continuous with the roots of the nerves, and can be seen entering the 

 chord passing towards the centre, and between the longitudinal fibres, but after 

 entering the gray substance are no longer traceable ; as shown in Fig. 14, Plate 

 I., some of them appear to join the longitudinal series, but were not traced far 

 enough to determine whether they did not ultimately join the gray substance in 

 their immediate neighborhood.f 



The gray substance, besides the white fibres which are intermixed with it, is com- 

 posed in part of cells more or less spherical or polygonal, such as are met with 

 in the brain, and, in addition, of distinct caudate cells, such as are represented in 



* Philosophical Transactions, 1851. 



t The evidence of the more recent microscopists tends to sliow that nearly all, perhaps all, of the 

 nerve roots enter the gray substance. A demonstration of this fact, with regard to the posterior roots, is 

 easily made on the spinal chord of a fcetal sheep, where they have the peculiarity of passing over the 

 outer surface of the posterior columns until they reach the median line, when (still on tlie exterior) they 

 descend into the fissure as far as the gray substance, in which they are wholly lost. Their course is 

 easily traced with a low doublet or even the naked eye. The posterior horn of gray substance reaches 

 the surface, and is traceable as a distinct band throughout the whole length of the chord. In the in- 

 stances which the writer has examined, none of the sensitive roots were traced into the posterior white 

 columns ; but all seemed to encircle them, and enter the gray substance at the bottom of the posterior 

 fissure. According to Mr. Clarke, the posterior roots in Man are traceable through the posterior columns 

 into the gray substance. 



