318 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



from its shape the fossa rhomboidea. Bisect the brain by a median sagittal cut. 

 Examine the cut surface, best under water. From the fourth ventricle a narrow 

 passage, the aqueduct of the brain, extends anteriorly. It communicates with the 

 cavity of the cerebellum, the cerebellar ventricle, and the cavities of the optic 

 lobes, the optic ventricles. The thick floor of the midbrain ventral to the aqueduct 

 forms the cerebral peduncles. The aqueduct opens into the cavity of the diencepha- 

 lon, the third ventricle. The roof of the diencephalon is very thin and consists of a 

 chorioid (vascular) plexus which is folded into the cavity. The anterior part of this 

 roof in the dogfishes extends dorsally into a sac, the paraphysis, resting against the 

 telencephalon. It is regarded as part of the latter. Posterior to the paraphysis, 

 a thin transverse partition, the velum transversum, is seen in the roof of the 

 diencephalon, particularly in the spiny dogfish. This marks the dorsal boundary 

 between diencephalon and telencephalon. The small thickened region of the 

 diencephalon just in front of the anterior end of the optic lobe is the habenula, 

 a smell center. From the habenula a slender process, the pineal body, may be 

 seen in favorable specimens, extending dorsally, just back of the paraphysis. 

 The entire roof of the diencephalon, including chorioid plexus, habenula, and 

 pineal body, is named the epithalamus and is concerned chiefly with smell. The 

 lateral walls of the diencephalon constitute the thalamus, an important correla- 

 tion center for various body senses. The ventral part of the diencephalon is 

 named the hypothalamus. It consists of the infundibulum, including the inferior 

 lobes, the hypophysis, and the mammillary bodies. The former should be identi- 

 fied on the section. The mammillary bodies are the thickened part of the ventral 

 wall above the vascular sac. The hypothalamus is concerned with smell and 

 taste. Note that the cavity of the third ventricle extends into all parts of the 

 hypothalamus. The third ventricle connects by a passage, the foramen of Monro, 

 or interventricular foramen, with the cavity in each half of the telencephalon. 

 (In the skate, the telencephalon is solid.) These two cavities are named the 

 first and second or lateral ventricles. They extend out into the olfactory bulbs 

 through the olfactory tracts. Cut into the telencephalon to see its ventricles. 

 Draw the sagittal section. 



C. THE NERVOUS SYSTEM AND SENSE ORGANS OF NECTTJRTJS 



i. The spinal nerves.— The spinal nerves are best found as follows. Make 

 a longitudinal cut along the side of the body below the lateral line. Cut through 

 the external and internal oblique muscles and separate this mass of muscle from 

 the thin layer of transverse muscles lying next to the coelom. The ventral rami 

 of the spinal nerves will now be seen running between the oblique and transverse 

 muscles, along the myosepta, and supplying the hypaxial muscles. Trace one 

 of these toward the vertebral column, cutting away muscles from its course. 

 It lies just behind the rib which may be cut away. The nerve may be traced 

 up to the vertebra where it is imbedded in an orange-colored material. On 



