3S8 MABEL BISHOP 



DESC'IUP'IMON OF ^rill-: NERVOUS SYSTEM 

 Spimd cord and spinal nerves 



The gross anatomy is best shown l)y the model ffigs. 4, 5, 6). 

 The brain and si)inal cord are weU developed, larger than in a 

 normal embryo of corresponding age, but not seemingly dispro- 

 portionate to the size of the teratological embryo. They present 

 no pathological or necrotic tissues or extrusions of brain sub- 

 stance such as are frequently found in abnormal embryos. 



The spinal coi'd in the region studied gives off from the outer 

 sides paired spinal nerves in normal manner and distribution. 

 But along the median ventral surface of the cord there is a promi- 

 nent neural ridge which diminishes gradually from about the 

 level of the cervical flexure to the mediastinum. From this 

 neural ridge there arise at regular intervals conjoined median 

 spinal nerves which are unganglionated and unbranched (figs. 5, 

 20). Cross-sections of the upper cervical cord (figs. 18 to 20) 

 indicate that the dorsal quadrants of the inner halves either do 

 not exist or are very greatly reduced in this region, therefore the 

 neural ridge consists chiefly of fused ventral quadrants of the 

 juxtaposed members. The median spinal nerves seem to be- 

 come successively smaller and to fade out after turning caudad 

 in the midline for a greater or less distance. They finally dis- 

 appear entirely and normal relations are maintained by spinal 

 nerves of the outer (normal) sides. 



Sympathetic nervous system 



In the cervical region. In the neck region of each normal side 

 a stout truncus sympatheticus lies parallel to the vagus nerve 

 between it and the normal spinal nerves (fig. 7). The truncus 

 curves dorsally around the ganglion nodosum and thus comes to 

 lie between the vagus and the ventral surface of the spinal cord. 



Fig. 4 External lateral view of the model of the brain and cerebrospinal 

 nerves of head B (i.e., of its left or normal side). The tongue of head B, which 

 lies in the median region, is added in diagram to show its innervation. The 

 numbers upon the perpendicular at the right indicate the levels of the sections 

 of the embryo represented in figures 8 to 20. 



