glandular mass containing peculiar cells which were origi- 

 nally thought to be sensory in function. It is now assumed 

 that they are endocrine. The pars nervosa and the anterior 

 lobe of the pituitary form a large mass lying somewhat an- 

 terior to the saccus vasculosus. Dorsolateral to the saccus are 

 the bilateral inferior lobes of the hypothalamus. The optic 

 nerves form a chiasma anterior to the inferior commissure 

 of the thalamus; in this decussation the two nerves cross one 

 above the other: one nerve passing through the other or 

 both nerves interlacing. 



The mesencephalon is represented by large paired optic 

 lobes, which are essentially hollow; anteriorly they touch 

 the telencephalon and posteriorly the cerebellum. The mes- 

 encephalic brain stem is not distinguished by any particular 

 structures from the rest of the lateral wall and floor. The 

 oculomotor nerve emerges from this part of the stem. 



The metencephalon has a large cerebellar mass dorsally. 

 This mass is extended forward as a tongue between the optic 

 lobes; this extension is the valvula cerebelli. The valvula is 

 folded upon itself in the salmon and projected into the 

 cavity of the mesencephalon. The decussation of the troch- 

 lear nerve passes below the posterior end of the valvula. In 

 the mormyrid fishes the valvula is extended outward above 

 the optic lobes as folded bilateral lobes. These cover much 

 of the brain. The stem portion of the metencephalon is not 

 marked from the rest of the brain stem, but from it arises 

 the fourth and perhaps a part of the fifth nerve. 



The membranous roof of the myelencephalon is modified 

 into a chorioid plexus. The vagal lobes are highly developed 

 in some teleosts and may meet above the fourth ventricle, 

 forming a tuberculum impar (Lampanyctus). From the sides 

 and ventral wall of the medulla arise nerves V to X. The 

 seventh and eighth nerves are represented by a common 

 group of roots. The tenth has two main roots: one lateral- 

 line sensory root the other a sensory and motor root serving 

 the branchial arches and the viscera. The sixth nerve arises 

 ventrally from the motor part of the myelencephalon. Pos- 

 terior to these, the spino-occipital nerves and finally the 

 spinal nerves arise. The spino-occipital nerves exit through 

 the cranial wall. The first spinal nerve lies between the skull 

 and the first vertebra. 



In the medulla there is a pair of giant neurons called 

 Mauthner's cells. The cell bodies lie at the vertical of the 

 roots of the seventh and eighth nerves near the midline; the 

 axons cross and descend through the spinal cord just below 

 the central canal. These giant fibers are observed in the larvae 

 of salamanders as well as actinopterygians. They function in 

 the coordination of swimming. In the adult of some teleosts 

 these cells are lacking. 



It would be difficult to describe all of the many variations 

 of brain pattern observed among the actinopterygians. 

 The most diagnostic feature appears to be the everted telen- 

 cephalic lobes. Other features are the usual lack of evagi- 

 nated olfactory bulbs and the occurrence of a valvula pro- 

 jected into the roof or cavity of the mesencephalon. The 



highly developed cerebellum and medulla is related to the 

 development of the lateral-line system in this group. 



The cranial nerves of the actinopterygian are similar to 

 those already observed (Figure 13-11). As in the dipnoans, 

 coelacanths, or the larval amphibians, there is a well-devel- 

 oped lateral-line system. The seventh nerve gives rise to a 

 superficial ophthalmic, buccal, and mental divisions. In the 

 catfish a branch of the seventh extends back on the body to 

 serve the "taste" buds which are scattered over much of the 

 surface of the anterior half of the body. The ninth and tenth 

 nerves have large sensory components. The tenth nerve, 

 which includes the spinal accessory nerve of the higher 

 forms, is here two-parted: one a lateral-line nerve and the 

 other a more typical vagoaccessory nerve. The "hypoglossal 

 nerve" arises from the branchial plexus formed from the 

 first two or three spinal nerves. 



It is interesting to note the common origin in development 

 of the seventh and eighth nerves. It is usually assumed that 

 they are parts of a single nerve. In some actinopterygians 

 the profundus division of the fifth is separated from the V2 

 and V3 division {Amia, Lepisosteus, and Polypterus), suggesting 

 that it is not a part of the trigeminal. A problem of termi- 

 nology is thus encountered regarding the C.asserian ganglion. 

 Should one refer to Gasserian profundus and Gasserian 

 maxillaris and mandibularis or should both parts receive 

 new names? 



Chondrichthyes 



Shark The nervous system of the shark, Sqimlus, is typical 

 of this group (Figure 13-12). The olfactory bulbs lie close to 

 the olfactory capsules; the olfactory nerve is made up of 

 many small bundles of fibers entering the bulb directly. The 

 bulb is connected with the cerebral hemispheres by a long 

 olfactory tract. The length of the tract is determined by the 

 elongation of the head. 



The cerebral hemispheres are relatively larger than those 

 of teleosts and have thick walls. The third ventricle extends 

 forward for half the length of the telencephalon. Anteriorly 

 the cerebral lobes are joined across the midline, and the 

 lateral ventricles extend anterolaterally into the olfactory 

 bulbs. Within each hemisphere the lateral ventricle expands 

 as a distinct chamber. Posteriorly the roof between the cere- 

 bral lobes is membranous and is extended outward as a 

 distinct paraphysis. The posterior end of the telencephalon is 

 marked by a transverse velum above and by the preoptic 

 recess below. 



The diencephalon has a membranous roof which is devel- 

 oped as a dorsal sac; the velum transversum gives rise to a 

 chorioid plexus. There is a long, thin epiphysis extending up- 

 ward from the posterior end of this roof. The epiphysis is 

 attached in the region of the habenulae, which lie just an- 

 terior to the posterior commissure. Ventrally there is an 

 optic chiasma and the hypothalamus is drawn out and back 

 as a distinct infundibulum ending in a saccus vasculosus. 



394 



THE NERVOUS SYSTEM 



