NERVOUS SYSTEM. (COMPARATIVE ANATOMY.) 



615 



is at once seen ; in the latter, the ganglia are 

 numerous and undetermined. We will notice 

 these parts separately. 



The spinal cord* (jig. 351, g,) is remarkable 

 for its great relative size in this class of animals : 

 it is continued (with but very few exceptions) 

 the whole length of the vertebral column, even 

 into the caudal vertebra, and it has on its an- 

 terior and posterior aspects a longitudinal fis- 

 sure (fig. 351, /(), the latter being the deepest; 

 internally it is hollowed out by a canal (i) 

 which traverses it in its whole extent, and 

 which, at the upper part, immediately posterior 

 to or underneath the cerebellum, forms a con- 

 siderable dilatation or enlargement the fourth 

 ventricle (Jig. 352, e). The posterior fissure 

 extends to this canal. In a river lamprey 

 (Petromyzonfluvialis), weighing 570 grains, 

 the brain weighs only four-tenths of a grain, 

 while the spinal cord weighs three grains, the 

 proportions being as 100 : 750. We thus ob- 

 serve how much the latter preponderates in 

 size, being seven and a half times heavier than 

 the brain. It is inclosed in a semicartilaginous 

 case, and I satisfactorily traced it into the ex- 

 treme point of the caudal extremity of the 

 animal : it presents a thin flattened appearance, 

 so much so that no trace of a central canal is 

 perceptible ; but immediately posterior to the 

 brain, the rudimentary corpora restiformia of the 

 two lateral longitudinal columns diverge to 

 form a large excavation, which is covered over 

 by a net-work of delicate vessels, a sort of 

 plexus choroides ; this is the fourth ventricle. 



Amongst the true osseous fishes I have found 

 a canal traversing the spinal marrow with this 

 dilatation or ventricle at its superior portion, 

 in the eel (Anguilla), perch (Percafiumalis), 

 gurnard ( Trlgla gurnardus), cod ( Gadus mo- 

 r/nia), mackarel (Scomber vulgaris ), pike 

 ( Esox Indus), roach ( Leuciscus rutilus), dace 



(Leuciscus vulgar is), chub (Leuciscus ?), 



carp(Cyprinuscarpio),andska.tefRaia ?) 



In the gurnard there are six pair of ganglia 

 developed on the superior surface, immediately 

 posterior to the cerebellum, at the origins of 

 the nerves distributed to the large pectoral fins ; 

 this remarkable conformation only exists in this 

 genus. In all the other species the spinal cord 

 is of nearly equal diameter throughout, except- 

 ing towards its termination ; and in the dace I 

 traced it running to the extremity of the tail, 

 and ending in a point : in the moon fish (Te- 

 trodon molu) it is remarkably short, and termi- 

 nates in a true cauda equina. 



[A similar exception to the usual length of 

 the spinal cord in fishes, is found in the Lophius 

 piscaturius, in which that organ ceases as high 

 as the eighth vertebra, and in one instance 

 observed by Leuret as high as the second. 

 The rest of the canal is occupied by cauda 

 equina.] 



The superior portion of the spinal cord, which 

 takes the name of medulla oblongata,is large and 



* In the description of the spinal cord the terms 

 anterior and posterior are used in the same signi- 

 fication as in the human subject ; anterior to signify 

 the surface next the bodies of the vertebrae, posterior 

 that next the spinous processes. 



broad in most fishes : on it are perceptible the 

 corpora pyramidalia and restiformia; the olivaria 

 are not yet developed. The former, situated on 

 either side of the anterior longitudinal groove, 

 are flattened and broad, and are distinctly seen 

 continuous with the crura cerebri, the pons 

 Varolii being wanting. The corpora restiformia, 

 or cerebellic fasciculi, are situated posteriorly ; 

 they separate (as before observed) at their upper 

 part to form the fourth ventricle, and pass after- 

 wards into the cerebellum. According to 

 Leuret there is no decussation of the fibres of 

 the spinal cord in fishes. 



[A singular little fish which has lately attracted 

 the attention of naturalists, and for the reception 

 of which Mr. Yarrell has instituted the genus 

 Amphioxus, exhibits the apparent anomaly of 

 an absence of all outward distinction between 

 the brain and spinal cord. It is the Amphioxus 

 Lanceolatus, of which a very perfect specimen 

 has lately been presented to the Museum of 

 King's College, London, by Professor Edward 

 Forbes. 



An elaborate examination of the anatomy 

 of this little creature has been published by 

 Mr. John Goodsir in the Transactions of the 

 Royal Society of Edinburgh, from which we 

 extract the following account of its neuro- 

 skeleton and of its nervous system. 



" Ncuru-skeleton. The osseous system, pro- 

 perly so called, consists of a "chorda dorsalis" 

 tapering at both ends, without the vestige of a 

 cranium, and of a dorsal and ventral series of 

 cells, the germs of superior and inferior inter- 

 spinous bones and fin rays. The " chorda 

 dorsalis " consists of sixty to seventy vertebra-, 

 the divisions between which are indicated by 

 slight bulgings, and lines passing obliquely from 

 above downwards on the sides of the column. 

 In this way a separation into individual ver- 

 tebra? is rather indicated than proved to exist; 

 for although the column has certainly a ten- 

 dency to divide at the points above-mentioned, 

 yet that division is rather artificial than natural. 

 There is no difficulty in ascertaining above 

 sixty divisions, those at each end above the 

 number stated run so much into one another 

 that no correct result can be obtained. 



" The chorda dorsalis is formed externally 

 of a fibrous sheath, and internally of an im- 

 mense number of laminae, each of the size and 

 shape of a section of the column at the place 

 where it is situated. When any portion of the 

 column is removed, these plates may be pushed 

 out from the tubular sheath, like a pile of coins. 

 They have no great adhesion to one another, 

 are of the consistence of parchment, and appear 

 like flattened bladders, as if formed of two 

 tough fibrous membranes pressed together. 



" As the fibres of the sheath are principally 

 circular, provision is made for longitudinal 

 strains on the column by the addition of a su- 

 perior and inferior vertebral ligament, as strong 

 cords stretching along its dorsal and ventral 

 aspects. The superior ligament lies imme- 

 diately under the spinal cord, and may be re- 

 cognized as a veiy tough filament, when the 

 column is torn asunder, or some of the ver- 

 tebra removed. The inferior ligament mav be 



