320 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



study its dorsal surface. The brain is covered by a membrane, the primitive 

 meninx, which is more or less divisible into the pia mater, a delicate pigmented 

 membrane adhering to the brain, and an outer dura mater, which is separated 

 from the skull by the peridural space. 



The most anterior portion of the brain consists of the two elongated lobes of 

 the telencephalon. These are very indistinctly subdivided into anterior olfactory 

 lobes and posterior cerebral hemispheres. The olfactory lobes include the bulbs 

 and tracts of the dogfish. Between and behind the posterior ends of the tel- 

 encephalic lobes is a thin roof containing a chorioid plexus. The anterior part 

 of this bears a process, the paraphysis, projecting forward in the groove between 

 the two cerebral hemispheres. It is regarded as a part of the telencephalon. 

 The very small region posterior to this is the roof of the diencephalon. It bears 

 a dorsally projecting process, the pinal body or epiphysis, which lies immediately 

 posterior to the paraphysis. Posterior to the diencephalon are the two elongated 

 optic lobes, representing the dorsal part of the mesencephalon. Behind this is 

 another chorioid plexus, a dark pigmented membrane. This may be removed. 

 There is then revealed the triangular cavity of the fourth ventricle. The anterior 

 end of this cavity is overhung by a narrow shelf at the posterior margins of the 

 optic lobes. This shelf is the cerebellum. The remainder of the brain consists 

 of the medulla oblongata which incloses the cavity of the fourth ventricle. The 

 medulla extends forward at the sides of the optic lobes as projections, the greater 

 part of which consists of the acustico-lateral area. The walls of the medulla are 

 divided into dorsal or sensory portions and ventral or motor portions, the latter 

 forming a broad band on either side of the median ventral groove. The further 

 subdivisions of these are not evident in Necturus. 



Draw the dorsal view of the brain. 



5. The cranial nerves. — 



a) The first or olfactory nerve: This is a stout band passing from the olfactory 

 sac to the anterior end of the telencephalon. 



b) The second or optic nerve: This small nerve may be seen in the floor of 

 the cranial cavity by pressing the telencephalon to one side. It passes obliquely' 

 caudad to the ventral surface of the diencephalon. 



c) The eye muscle nerves: The third or oculomotor, fourth or trochlear, and 

 sixth or abducens nerves are so small in Necturus as to be scarcely discernible in 

 gross dissection. They originate from the same regions of the brain and supply 

 the same eye muscles as in elasmobranchs. 



d) The fifth or trigeminus nerve: The fifth nerve is the large trunk arising 

 from the side of the anterior end of the medulla. Trace it out through the 

 skull. It passes in front of the otic capsule and immediately enters a large 

 ganglion, the semilunar or Gasserian ganglion of the trigeminus. From the 

 ganglion three large nerves are given off, the ophthalmic, maxillary, and man- 

 dibular branches. The former, corresponding to the deep ophthalmic of fishes, 

 passes forward through the quadrate cartilage and runs forward anteriorly along- 



