NERVOUS SYSTEM 



861 



The flexures are never very well marked and disappear entirely in 

 the adult. 



PISCES 



The olfactory organs are paired and end blindly, not communicating: 

 with the pharynx as in terrestrial animals and hagfishes. The auditory 

 organs are entirely internal, and have no communication with the ex- 

 terior. They serve largely as organs of equilibration, though they also- 

 receive vibrations. The eyes are much like those of other vertebrates, 

 except that they are lidless and have spherical lenses of short range 

 vision in the water. The brain is small and shows no fissures. It never- 

 theless has all the characteristics of the vertebrate brain, though there 

 are but ten cranial nerves (Fig. 482). The spinal cord is like that in 

 other vertebrates. 



DOGFISH 



Although the brain is very small and compact, it is larger in propor- 

 tion to body size than that of the cyclostomes. The most striking feature 

 is the large size of the olfactory lobes, and the slight development of the 

 intercerebral fissure. The cerebral hemispheres are well defined, the 

 cerebellum is large, and overlaps anteriorly a part of the optic lobe, and 

 posteriorly a part of the medulla oblongata. The corpora restiformia 

 are large folds on each side of the cerebellum in front and lateral to the 

 rhomboid fossa. The region of the thalamencephalon from which the 

 optic nerve springs is comparatively small and slender. The spinal cord is 

 typical and enclosed within cartilaginous neural arches. The dominant 

 sense of the dogfish is olfactory, the sense organs consisting of large con- 

 voluted invaginations in close contact with the olfactory lobes of the 

 brain. The eyes, although small and probably not especially keen- 

 sighted, are well developed and connected within the brain by a rather 

 slender optic nerve. The auditory organs are enclosed in cartilaginous 



Zphpr 



Fig. 482. 



Cranial Nerves of the Fish. (Schematic.) ev., spiracle; mand., 

 mandibular branch of the V; max., maxillary branch of the V; m.t., 

 masticator branch of V ; m., neural cord ; oph.pr., deep ophthalmic branch 

 of the V; ph., pharyngeal branches of branchial nerves ; pot., post- 

 trematic branch ; pt., pretrematic branch ; pl.c., cervico-branchial plexus ; 

 r.pal. VII, palatine branch of the VII; r.pal. IX, palatine branch of the 

 IX; sp, spinal nerves; sp.o., spinal-occipital nerves; V to X, pairs of the 

 corresponding cranial nerves; 1 to 4, branchial slits. (From Vialleton.) 



