NERVOUS SYSTEM. VERTEBRA! A. 



PISCES. 

 CYCLOSTOMI. 



D. 749. Two pieces of the trunk of ;i Sea-Lamprey 



marinus) with the cord exposed from ahove in on*-, and in 

 transverse section in the other, to show its dorse-ventral I v 

 flattened, ribbon-like form. The grey matter of tli. 

 surrounds the central canal and has the same outline in 

 cross-section as the cord itself. In the white matter tire a 

 number of large fibres (Miiller's fibres). The cord occupi-> 

 only a small part of the neural canal, and is surrounded by 

 two fibrous sheaths an outer dura mater attached here and 

 there by trabeculse to the fibrous walls of the canal, ami a 

 delicate pia mater close to the cord. Between the two 

 there is a large subdural space occupied by loose trabecular 

 tissue. In the upper specimen, the cord is seen in 

 transverse section situated near the floor of the neural canal 

 and surrounded by a mass of subdural tissue. In the lower 

 specimen the two sheaths have been slit along the dorsal 

 mid- line, and black paper has been inserted beneath them. 

 A red rod has been placed beneath the posterior part of 

 the cord. 



ELASMOBEANCHIL 



D. 750. A section of the head and vertebral column of a Skate 

 (Raia batis), in which the cerebro-spinal axis has )-. n 

 laid bare by the removal of the posterior walls of the 

 cranial and spinal cavities. The spinal cord is continued 

 to the caudal end of the vertebral column, regularly and 

 very gradually decreasing in size. The dorsal root.- <>l 

 the spinal nerves can be seen arising from the dorso-lateral 

 borders of the cord at regular intervals and passing diagon- 

 ally backwards to their point of exit from the neural canal, 

 their direction becoming more and more oblique toward- 

 the hinder end of the body. In the anterior region 

 the dorsal roots occur at more frequent intervals than 

 further backhand here perforate the continuous cartilaginous 

 wall? of the neural canal. When separate vertebral arches 

 differentiated (about 80 mm. behind the skull) a dond 



are 



