616 



NERVOUS SYSTEM. (COMPARATIVE ANATOMY.) 



Fig. 350. 



raised from the inferior 

 surface of the column in 

 the form of a tough rib- 

 bon. From the sides of 

 the column aponeurotic 

 laminae pass off to form 

 septa of attachment be- 

 tween the muscular bun- 

 dles; and along the me- 

 sial plane above the co- 

 lumn, a similar lamina 

 separates the superior bun- 

 dles of each side, and by 

 splitting below and run- 

 ning into the sides of the 

 column, forms a fibrous 

 canal for the spinal cord. 

 Foramina exist all along 

 the sides of this canal for 

 the passage of the nerves. 

 A similar septum is situ- 

 ated along the inferior part 

 of the column, from the 

 part where the inferior 

 muscular bundles unite at 

 the anus, to the extremity 

 of the tail. Along the 

 superior edge of the apo- 

 neurotic septum, between 

 the dorsal muscular bun- 

 dles, and stretching from 

 the anterior point of the 

 vertebral column to a point 

 beyond the anus, and half 

 embedded between the su- 

 perior extremities of the 

 muscles, is a series of 

 closed cells of a flattened 

 cylindrical form, adhering 

 firmly to one another by 

 their bases, so as to pre- 

 sent the appearance of a 

 tube flattened on the sides 

 with septa at regular dis- 

 tances. Each of these 

 cells is full of a trans- 

 parent fluid, in the centre 

 of which is an irregular 

 mass of semi-opaque glo- 

 bules, apparently cells. 

 This series of cylindrical 

 sacs consists of the ru- 

 diments of inter- spinous 

 bones, and probably of fin 

 rays, and is attached be- 



The nervous system o/low to the fibrous inter- 

 Amphioxuslanceolatus. muscular septa, half co- 

 a,a, the spinal cord ; vered on each side by the 

 b, the first pair of lateral muscles, and en- 

 nerves ; c, the dorsal ; c i ose( j above by the tegu- 

 oht%r o nd pat" h -ntary fold which con- 



stitutes the dorsal tin. 



" A similar series of cells, with the same 

 relations, is situated on the ventral surface of 

 the body, and stretches from the spot where 

 the abdominal folds terminate, to a point 

 nearly opposite the termination of the dorsal 

 series. 

 " Nervous system. The spinal cord is 



situated on the upper surface of the chorda 

 dorsalis, enclosed in the canal formed in the 

 manner above described. When the whole 

 length of this canal is displayed by removing 

 the muscles, and then carefully opened, the 

 spinal cord is seen lying in the interior, with 

 nerves passing out from it on each side. It 

 stretches along the whole length of the spine, 

 is acuminated at both ends, and exhibits not 

 the slightest trace of cerebral development. In 

 its middle third, where it is most developed, it 

 has the form of a ribbon, the thickness of which 

 is about one-fourth or one-fifth of its breadth ; 

 and along this portion, also, it presents on its 

 upper surface a broad, but shallow groove. 

 The other two-thirds of the cord are not so flat, 

 and are not grooved above, are smaller than 

 the middle third, and taper gradually; the one 

 towards the anterior, the other towards the 

 posterior extremity of the vertebral column. 

 A streak of black pigment runs along the 

 middle of the upper surface of the cord. It 

 is situated in the groove already described, 

 and is in greater abundance anteriorly and pos- 

 teriorly, where the nerves pass off at shorter 

 intervals, than at the -middle or broadest part 

 of the organ. From fifty-five to sixty nerves 

 pass off from each side of the cord ; but, as 

 the anterior and posterior vertebrae are very 

 minute, and run into one another, and as the 

 spinal cord itself almost disappears at the two 

 extremities, it is impossible to ascertain the 

 exact number, either of vertebrae or of spinal 

 nerves. These nerves are not connected to the 

 spinal marrow by double roots, but are inserted 

 at once into its edges in the form of simple 

 cords. 



" The nerves pass out of the intervertebral 

 foramina of the membranous spinal canal, 

 divide into two sets of branches, one of which 

 run up between the dorsal muscular bundles 

 (dorsal branches) ; the other (ventral branches) 

 run obliquely downwards and backwards on 

 the surface of the fibrous sheath of the vertebral 

 column ; attach themselves to the antero-pos- 

 terior aspect of each of the inferior muscular 

 bundles, and may be distinctly traced beyond 

 the extremity of each bundle. When an entire 

 animal is examined by transmitted light, and a 

 sufficient magnifying power, the anterior extre- 

 mity of the spinal cord is observed, as before 

 mentioned, to terminate in a minute filament 

 above the anterior extremity of the vertebral 

 column. The first pair of nerves is excessively 

 minute, and passes into the membranous parts 

 at the anterior superior angle of the mouth. 

 The second pair is considerably larger, and, 

 like the first pair, passes out of the canal in 

 front of the anterior muscular bundle. The 

 second pair immediately sends a considerable 

 branch (corresponding to the dorsal branches 

 of the other nerves) upwards and backwards, 

 along the anterior edge of the first dorsal mus- 

 cular bundle. This branch joins the dorsal 

 branch of the third pair, and, passing on, joins 

 a considerable number of these in succession, 

 and at last becomes too minute to be traced 

 farther. After sending off this dorsal branch, 

 the second pair pusses downwards and back- 



