THE SPECIAL ANATOMY OF THE FROG 47 



olfactory lobe continues forward to the nasal sac as the olfactory nerve. The 

 posterior boundary of the olfactory lobes is marked by a transverse groove which 

 separates them from the next part of the brain, a pair of long oval bodies, the 

 cerebral hemispheres. Just behind the cerebral hemispheres is a depressed region, 

 the diencephalon or thalamencephalon, from which a delicate stalklike body, the 

 pineal body, ascends dorsally to the brow spot. The pineal body is almost always 

 torn off in removing the skull and hence cannot be seen. Posterior to the 

 diencephalon are the two rounded optic lobes. Just behind them and forming 

 the anterior wall of the triangular cavity of the fourth ventricle is the cerebellum. 

 This is much smaller in the frog than in most other vertebrates. The region 

 of the brain posterior to the cerebellum, and forming the floor and lateral walls 

 of the fourth ventricle is known as the medulla oblongata. It narrows posteriorly 

 until it becomes of the same width as the spinal cord with which it is continuous. 



2. The cranial nerves (Holmes, p. 295). Ten pairs of nerves spring from 

 the lateral and ventral surfaces of the brain but most of these cannot be found 

 unless very great care is exercised in dissecting. The first pair of cranial nerves, 

 the olfactory, has already been noted arising from the olfactory lobes. The 

 second pair of nerves, the optic nerves, springs from the ventral surface of the 

 diencephalon, and may be seen by gently pushing this part of the brain to one 

 side. Each penetrates the adjacent eyeball where it forms the retina. The 

 third, fourth, and sixth nerves are motor nerves to the muscles of the eyeball 

 and are too small to be found. The fifth, seventh, and eighth nerves arise close 

 together from the side of the anterior end of the medulla, and may usually be 

 seen by gently pressing back this region. The ninth and tenth arise together 

 from the side of the medulla a short distance behind the eighth. 



3. The spinal cord. The spinal cord is continuous with the medulla oblongata 

 and occupies a cavity within the vertebral column known as the neural canal. 

 Posteriorly the spinal cord tapers into a fine thread, the filum terminale, which 

 occupies the cavity of the urostyle and can be seen by cutting off the dorsal half 

 of the urostyle. The spinal cord is slightly enlarged opposite the fore limb 

 (brachial enlargement} and again anterior to the filum terminale (sciatic enlarge- 

 ment}. These swellings are, of course, caused by the origin of the nerves to the 

 limbs at those regions. In the median dorsal line of the cord is a longitudinal 

 groove, the dorsal fissure. In a well-di'ssected specimen, the dorsal roots of the 

 spinal nerves may be seen arising at regular intervals from the dorso-lateral 

 region of the cord. The posterior roots run within the neural canal for a little 

 distance alongside the spinal cord before passing out of the vertebral column. 



Draw the brain and cord from the dorsal side. 



4. Ventral aspect of the brain. Cut across the cord back of the medulla 

 oblongata, and carefully remove the brain, leaving the cord in place. In remov- 

 ing the brain note and cut through the more conspicuous of the cranial nerves. 

 Study the ventral surface of the brain and identify the parts already noted on 



