NERVOUS SYSTEM. (COMPARATIVE ANATOMY.) 



617 



wards on each side above the hyoid apparatus, 

 and joins all the ventral branches of the other 

 spinal nerves in succession, as its dorsal branch 

 did along the back. This ventral branch of 

 the second pair is very conspicuous, and may 

 be easily traced along the line formed by the 

 inferior extremities of the ventral divisions of 

 the muscular bundles, the ventral branches of 

 the other nerves joining it at acute angles 

 between each bundle. It may be traced be- 

 yond the anus, but is lost sight of near the 

 extremity of the tail. Twigs undoubtedly pass 

 from the spinal and lateral nerves towards the 

 abdominal surface of the body, but, on account 

 of their minuteness, and the difficulty of de- 

 tecting them in detached portions of the 

 abdominal membrane, they could not be satis- 

 factorily seen. 



" When a portion of the spinal cord is 

 examined under a sufficient magnifying power, 

 it is seen to be composed entirely of nucleated 

 cells, very loosely attached to one another, but 

 enclosed in an excessively delicate covering of 

 pia mater. The cells are not arranged in any 

 definite direction, except in the middle third 

 of the cord, where they assume a longitudinal 

 linear direction, but without altering their 

 primitive spherical form. The black pigment, 

 formerly mentioned as existing more particu- 

 larly on the upper surface and groove, is 

 observed to be more abundant opposite the 

 origin of the nerves ; and, as it is regularly 

 arranged in this manner in dark masses along 

 the anterior and posterior thirds of the cord, 

 the organ in these places, on superficial inspec- 

 tion, resembles much the abdominal ganglionic 

 cord of an annulose animal. Along the middle 

 third the pigment is not so regular, but appears 

 in spots at short intervals. When any portion 

 of the cord, however, is slightly compressed, 

 and microscopically examined, it becomes evi- 

 dent that there is, along the groove and mesial 

 line of its upper surface, a band, consisting of 

 cells of a larger size than those composing the 

 rest of the organ. Some of these cells only 

 are filled with black pigment, but all of them 

 contain a fluid of a brown tint, which renders 

 the tract of large cells distinctly visible. When 

 the compression is increased the cells burst ; 

 and the fluid which flows from the central 

 tract is seen to contain jet-black granules, 

 which may be detected as they escape from the 

 cells. 



" The nerves consist of primitive fibres, of 

 a cylindrical shape, with faint longitudinal 

 striae. The primitive fibres of a trunk pass off 

 into a branch, in the usual way, without 

 dividing; and, where the trunks join the spinal 

 cord, the primitive fibres are seen to approach 

 close to it, but without passing into it. The 

 greater part of the slightly protuberant origin 

 consisting of the nucleated cells of the cord, 

 with a few pigment cells interspersed, the 

 exact mode of termination of the central ex- 

 tremities of the primitive nervous fibres could 

 not be detected." 



We hope we may be excused for quoting the 

 following additional remarks. 



" One of the most remarkable peculiarities 



in the Lancelet is the absence of the brain. 

 Retzius, indeed, describes the spinal marrow 

 as terminating considerably behind the anterior 

 extremity of the chorda dorsalis, in a brain 

 which exhibits scarcely any dilatation; but 

 careful examination of the dissection of my 

 own specimen, which I have also submitted to 

 the inspection of Dr. John Reid, and of other 

 competent judges, has convinced me that the 

 spinal cord, which may be traced with the 

 greatest ease to within 1-1 6th of an inch of 

 the extremity of the chorda dorsalis, does not 

 dilate into a brain at all. It may be urged that 

 we ought to consider the anterior half of the 

 middle third of the spinal marrow, where it is 

 most developed, to be the brain, and all that 

 portion of the chorda dorsalis which is in con- 

 nection with the branchial cavity, as the cra- 

 nium. That this does not express the true 

 relation of the parts, is evident from the fact 7 

 that this portion of the cord, to its very extre- 

 mity, gives off nerves, which are too numerous 

 to be considered as cerebral, but more espe- 

 cially from the mode of distribution of the 

 first and second pairs, which, in my opinion, 

 proves the anterior pointed extremity to be the 

 representative of the brain of the more highly 

 developed vertebrata. A brain of such sim- 

 plicity necessarily precludes, on anatomical 

 grounds alone, the existence of organs of vision 

 and of hearing. These special organs, deve- 

 loped in the vertebrata at least, in a direct 

 relation with the cephalic integuments and the 

 brain, could not exist, even in the form of 

 appreciable germs, in the Lancelet. The black 

 spot which Retzius took for the rudiment of an 

 eye may probably have been, what also deceived 

 me at first, a portion of the black mud which 

 floats about in the branchial cavity, and which 

 adheres obstinately to the parts in the neigh- 

 bourhood of the oral filaments. The first pair 

 of nerves, although very minute, in accordance 

 with the slight development of the parts about 

 the snout, and the want of special organs of 

 sense, might, from their position and relations, 

 be considered as corresponding to the trifacial 

 in the higher vertebrata. The second pair 

 appears to be the vagus, not only from its 

 distribution as a longitudinal filament on each 

 side of the body, as in other fishes, but also 

 from its relations to the hyoid apparatus and 

 branchial cavity, to which division of organs 

 the eighth pair of fishes is specially devoted. 

 The distribution of a branch of this nerve, 

 however, along the base of the dorsal fin, and 

 the course of the posterior part of the main 

 branch, would appear to shew that this nerve, 

 which I have provisionally denominated the 

 vagus, is, in fact, the trifacial, which, in the 

 higher fishes, is not only distributed to all the 

 fins, but holds exactly the same relations to the 

 dorsal and anal fins, and to the spinal nerves, 

 as the nerve now under consideration in the 

 Lancelet. 



" The peculiarities in the structure of the 

 spinal cord are not less remarkable than those 

 of its configuration. It is difficult to under- 

 stand, according to the received opinions on 

 the subject, how a spinal cord destitute of 



