32 LABORATORY STUDIES IN ZOOLOGY 



the pia. In all probability the outer menix (covering or enve- 

 lope), the (J lira, has been more or less torn in removing the bone, 

 since in the frog it appears more as a lining of the cranial cavity 

 than as an envelope of the brain. The pigmented state of the 

 pia is especially noticeable in the caudal portion of the brain, 

 where it is rich in blood vessels and roofs over a triangular cavity. 

 With the forceps or needles remove the pia. 



For descriptive purposes the brain will be regarded as compris- 

 ing divisions and parts as follows: 



1. The forehrain exhibiting two divisions. The more cephalic 

 portion consists of a pair of rounded olfactory lohes, which are 

 separated by a faint mesal groove and are continued cephalad 

 as the "olfactory nerves" which extend to the organs of smell 

 located in the nasal canal. The caudal boundary of the olfac- 

 tory lobes is marked by a slight transverse depression, behind 

 which is to be seen the other portion of the forebrain, namely, 

 a pair of long oval bodies, the forebrain hemispheres. 



2. Just caudad of the hemispheres is a constricted region, the 

 twixt brain or interhrain, bearing in the midline and near the cau- 

 dal end of this region a short stalk, the pineal body. In the 

 undisturbed state this is found to extend to the bony roof of the 

 cranium where it ends in a knoblike enlargement. In the tadpole 

 (young frog) this outgrowth appears to be connected with that 

 area already identified on the dorsum of the head as the brown 

 spot which is looked upon as marking the location of a former 

 sense organ (or organs). The pineal stalk of the brain represents 

 its former nervous connections. 



3. Caudad of the interbrain is the midbrain of which the optic 

 lobes are the prominent features of the dorsal aspect. 



4. Just behind the optic lobes is a narrow, transverse ridge, 

 the cerebellum. This division in many other animals is larger 

 and an important center of equilibrium. In the frog it is not 

 extensively developed. 



5. The division caudad of the cerebellum is the oblongata or 

 medulla oblongata. Its dorsal wall is a thin, vascular membrane 

 consisting of pia and endyma (the lining of the cavity of the 

 brain) . In the roof of this division nervous substance disappears. 

 The vascular membrane thus formed is known as the posterior 

 choroid plexus. 



The Spinal Cord. — The spinal cord is continuous with the 

 oblongata and occupies a space within the vertebral column 



