822 



Comparative Animal Physiology 



the cord, and an increase in the number of spinal tracts to and from the 

 brain. 



Many afferent fibers cross or connect with arcuate interneurones. In cy- 

 clostomes there are ascending bundles of short fibers, and most spinal cor- 

 relation can be carried out in a few segments; the principal descending fibers 

 are of the reticular system. In elasmobranchs and teleosts ascending spino- 

 bulbar (medullary) and spino-mesencephalic tracts are present; in teleosts 

 spino-cerebellar tracts and the sympathetic ganglion chains appear, and the 



mmw'm 



Spinal Dogfish 



I Pressur- 



Time-5sec. 



Fig. 307. A, Rhythmic undulations in dorsal fin of a spinal dogfish; rhythm increased 

 when body touched lightly behind point of transection. B, Rhythmic movement of dorsal 

 fin in a spinal dogfish; brief struggle followed by inhibition when strong pressure was 

 applied by a clamp to the body. From Gray and Sand.^'" 



descending vestibulo-spinal tract is added. As limbs appear, the cord enlarges 

 in cervical and lumbar regions. Amphibans show a well defined central grey 

 H and great reduction in the number of spinal segments. In birds a spe- 

 cific tectospinal tract appears, whereas in lower vertebrates the tectum (part 

 of midbrain) connects with the cord via the reticulospinal system. In mam- 

 mals the corticospinals, particularly the pyramidals, appear as dominant 



