308 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



vessels, called a chorioid plexus. This particular plexus is named the chorioid 

 plexus of the third ventricle. The diencephalon serves chiefly as a center of 

 co-ordination of the principal sensations, such as sight, hearing, and skin sensa- 

 tions, its ventral portions being devoted to smell and taste. The diencephalon 

 is in lower vertebrates the chief controlling region of the brain. The optic nerve 

 passes from the orbit into the ventral surface of the diencephalon. It is quite 

 conspicuous in the skate and will be seen in the dogfishes by gently pressing the 

 diencephalon to one side. Posterior to the diencephalon is the midbrain or 

 mesencephalon, consisting dorsally of two rounded lobes, the optic lobes or corpora 

 bigemina. The optic lobes are centers for the visual, auditory, and general skin 

 sensations, and these are connected with corresponding centers in the 

 diencephalon. A nerve, the trochlear or fourth cranial nerve, arises from the 

 posterior borders of the optic lobe on each side and passes forward to an eye 

 muscle. By gently pressing the optic lobes to one side, a nerve, the oculomotor 

 or third cranial nerve, will be seen emerging from the ventral surface of the mid- 

 brain and passing to the orbit. 



Posterior to the optic lobes and somewhat overhanging them is the large 

 cerebellum or metencephalon. In the spiny dogfish and skate this is slightly divided 

 into four quadrants by faint longitudinal and transverse grooves; in the smooth 

 dogfish it is marked by several transverse grooves. The cerebellum is a center 

 for the co-ordination of motor impulses, including the maintenance of equilib- 

 rium. No nerves are attached to the cerebellum. Posterior to the cerebellum 

 the elongated remaining section of the brain is the medulla oblongata or myelen- 

 cephalon. It is continuous posteriorly with the spinal cord. The anterior part 

 of the roof of the medulla is thin and discolored and consists of a chorioid plexus, 

 that of the fourth ventricle. The anterior end of the medulla extends forward 

 at the sides of and below the cerebellum as two earlike projections, sometimes 

 called the auricles. Remove the chorioid plexus from the roof of the auricles 

 and the medulla. The large cavity revealed is the cavity of the fourth ventricle. 

 The auricles together with the portion of the dorsal part of the medulla just 

 posterior to them constitute the acustico-lateral areas, or primary centers of the 

 lateral line system and of hearing and equilibration. On lifting the posterior 

 end of the cerebellum it will be seen that the auricles are continuous with the 

 cerebellum and with each other, below the overhanging posterior portion of the 

 cerebellum. By way of this connection with the cerebellum the impulses which 

 come in from the lateral line system and internal ear are conveyed to the cere- 

 bellum after making a relay in the acustico-lateral area. The entire dorsal 

 rim of the medulla, including the acustico-lateral areas, forms an elongated 

 strip on each side which is known as the somatic sensory column. This column 

 as its name implies contains centers and tracts associated with all of the body 

 senses except smell, taste, and vision, which as we have seen are disposed of 



