378 ON THE ANATOMY OF AMPHIOXUS LANCEOLATUS. 



Nervous Sysh 



The spinal cord is situated on the upper surface of the 

 chorda dorsalis, euclosed 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 posteriorly, 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 im- 

 possible to ascertain the exact number, either of vertebras 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 



