268 ANIMAL PHYSIOLOGY. [LESSON 82. 



principally the nerves to the tentacula (/, /), and the two me- 

 dian ones (g,g) are connected with a ganglion (h,h), which supplies 

 the tentacula of the inferior labial processes, and the lamellated 

 organs on that part of the oral sheath. The tentacula nerves are 

 continued, like those of the arms in the higher cephalopods, along 

 the middle of the tentacle, attached by loose cellular tissue to the 

 vessels of the part. The posterior collar gives off numerous nerves 

 (m) of a flattened form, which supply the muscles of the shell. The 

 respiratory nerves form a small ganglion (q) at the base of each pair 

 of gills, from which branches are sent to those organs, and to the 

 heart. A plexus of more delicate visceral nerves (r) is continued 

 backwards along the interspace of the branchial nerves, and the chief 

 branches are connected with a small ganglion situated upon the 

 stomach ; the ophthalmic tentacula derive their nerves (n^ n) from the 

 immediate vicinity of the origin of the optic ganglion. The hol- 

 low plicated process beneath the eye, regarded by some authors as 

 the olfactory organ, likewise receives its nerves from the extremity 

 of the supra-cesophageal chord. 



LESSON LXXX1I. 



NERVOUS SYSTEM IN THE FISHES. 



1205. The great axis of the nervous system occupies entirely a 

 dorsal (dorsum, back) position in the vertebrated classes ; it is en- 

 closed in an osseous (bony) sheath or spine, which is continued over 

 its posterior prolongation, and it is nowhere perforated by the ali- 

 mentary canal. 



The fibrous structure of the encephalic portion (brain) which is 

 perceptible in the Cephalopods, becomes more distinct and obvious as 

 we ascend through the vertebrated classes ; and that anterior part 

 of the nervous axis becomes likewise proportionally larger, leaving 

 only slight traces, in the fourth ventricle, of its original opening for 

 the passage of the alimentary canal. 



The spinal chord, the medulla oblongata, the optic lobes, the 

 cerebral, and cerebellic hemispheres, form the most constant ele- 

 ments of thJfc axis, but their relative and actual developments vary in 

 the different classes. Though much varied in the extent of its de- 

 velopment in the different classes, there is great similarity in the 

 successive stages of the development of this system in the embryos 



