252 MR GOODSIR ON THE ANATOMY OF AMPHIOXUS LANCEOLATUS. 



where the nerves pass oft' 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 an- 

 other, 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 in- 

 serted at once into its edges in the form of simple cords. 



The nerves pass out of the intervertebral foramina of the membranous spinaJ 

 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-posterior 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 extremity 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 consider- 

 able branch (corresponding to the dorsal branches of the other nerves) upwards 

 and backwards, along the anterior edge of the first dorsal muscular bundle. This 

 branch joins the dorsal branch of the third pair, and, passing on, joins a consider- 

 able number of these in succession, and at last becomes too minute to be traced 

 farther. After sending off this dorsal branch, the second pair passes downwards 

 and backwards 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 beyond 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 ac- 

 count of their minuteness, and the difficulty of detecting them in detached por- 

 tions of the abdominal membrane, they could not be satisfactorily 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 



