72 VERTEBRATE ZOOLOGY 



The sacculus, which forms the ventral portion of the vestibule, 

 contains a large white otolith. At its hinder end is a small pro- 

 jection called the lagena, which is homologous to the cochlea of 

 higher vertebrates. 

 Exercise 9. Draw the parts of the ear, so far as observed. 



The Central Nervous System. Remove the skin and muscles 

 from the dorsal side of the head and the anterior end of the trunk. 

 Carefully shave off the bone and cartilage forming the roof of the 

 skull and expose the brain. Remove in the same way the dorsal 

 portion of the neural canal and expose the spinal cord. Remove 

 the membrane (the dura mater) which covers the brain. 



Without removing or disturbing it, study the dorsal surface of 

 the brain. It is made up of five regions of very different size,— 

 the cerebrum, the diencephalon, the optic lobes or midbrain, the 

 cerebellum, and the medulla oblongata. 



The anterior and largest division of the brain is the cerebrum, 

 which is composed of two hemispheres ; no distinct olfactory lobes 

 are present. Back of the cerebrum is a large median structure 

 made up of the diencephalon and the midbrain, no line of de- 

 marcation between them being present. Projecting forward from 

 the diencephalon between the hemispheres is the large pineal 

 body, or epiphysis. 



Directly continuous with the midbrain posteriorly, and sepa- 

 rated from it by a shallow groove, is the cerebellum, a narrow, 

 transverse ridge which forms the dorsal portion of the fourth 

 division of the brain. Immediately back of it is the medulla ob- 

 longata, the hindermost division, which is continuous with the 

 spinal cord. On its dorsal surface is a triangular depression, the 

 fossa rhomboidalis, which marks the position of the fourth ven- 

 tricle of the brain. 



Exercise 10. Draw the dorsal aspect of the brain and a portion of the 

 spinal cord on a scale of 2 or 3. 



The Cranial Nerves. Ten pairs of nerves spring from the brain 

 and pass to the organs and tissues of the head and the anterior 

 part of the trunk. Cut away the side of the skull .and expose the 

 side of the brain ; find the cranial nerves. 



