17 



also be cut away from above the portion of the spinal cord next 

 the skull. Gently wash away any coagulated lymph. 



The brain and spinal cord are invested by two membranes 

 (meninges) . The tough dura mater lines the cavity in which they 

 lie, clinging closely to the cartilage; in fact it forms the perichon- 

 drium of the internal surface of the cranium. The pia mater en- 

 velops closely the brain and cord, and contains numerous blood 

 vessels. Between the two is the arachnoid space, traversed by oc- 

 casional fine threads of connective tissue and filled with lymph. 



As the spinal cord passes forward into the skull it enlarges and 

 merges with the posterior portion of the brain, the medulla oblon- 

 gata (myelencephalon) . The roof of the medulla is extremely 

 thin, and is broken if the cartilage has not been removed with ex- 

 treme care, exposing a cavity within, the fourth ventricle. 



In front of the medulla, and overlapping its anterior extremity, 

 is a large oval organ, the cerebellum (metencephalon) . Ventral 

 to the cerebellum, each side of the medulla is expanded in an ear- 

 shaped lobe, the corpus restiformis. Anteriorly, the cerebellum 

 overlaps a pair of rounded lobes, the optic lobes, which together 

 form the dorsal portion of the midbrain (mesencephalon) . 



In front of the optic lobes are two slightly larger lobes united 

 in their posterior portions but separated anteriorly, the cerebral 

 lobes or hemispheres. Together they constitute the prosencephalon. 

 (The prosencephalon is not divided in Eugaleus.) Between the 

 mesencephalon and the prosencephalon is a depressed region be- 

 longing to the brain-stem, the diencephalon (thalamencephalon) , 

 from which the epiphysis arises. The roof of the diencephalon 

 also is very thin and is frequently broken during the exposure of 

 the brain. The cavity seen within the diencephalon is the third 

 ventricle. 



Stalked bodies arising from the antero-lateral angles of the 

 cerebral hemispheres are the olfactory lobes. The portion of the 

 brain including the cerebral hemispheres and the olfactory lobes 

 constitutes the telencephalon. 



DISSECTION OF THE INTERNAL EAR. The structures composing 

 this organ lie in the projecting cartilage at the side of the medulla 

 (auditory capsule). Remove the cartilage of the auditory capsule 

 in thin slices and bit by bit, following the ductus endolymphaticus 

 to the membraneous labyrinth. Dissect away the surrounding car- 

 tilage leaving the membraneous canals in place, until the entire 

 labyrinth is exposed. The membraneous labyrinth consists of a 

 large central sac (utriculo-saccular chamber) into which the endo- 

 lymphatic duct opens, and three membraneous tubes (semicircular 

 canals) external to the chamber but communicating with it in vari- 

 ous ways. Two, one anterior and the other posterior to the sacculus, 

 lie in a nearly vertical plane (anterior and posterior semicircular 

 canals) ; one is external to the sacculus and lies in a nearly hori- 

 zontal plane (horizontal or external semicircular canal). At the 

 ventral ends of the vertical canals are nearly spherical enlarge- 

 ments called ampullae. The ampulla of the horizontal canal is at 

 its anterior end. The dorsal ends of the vertical canals open near 

 each other into the upper part of the utriculo-sacculus. The ven- 

 tral extremity of the anterior vertical canal and the anterior ex- 

 tremity of the horizontal canal open beside each other into an an- 

 terior projection of the sacculus. The ventral extremity of the pos- 

 terior vertical canal opens into the posterior and lower part of the 

 sacculus. The posterior extremity of the horizonal canal opens 



