296 ESSENTIALS OF ZOOLOGY 



side : an anterior leading to the crura, a middle to the pons, and 

 a posterior to the medulla and spinal cord. The nerve cells of 

 the cerebellum and their associations with the vestibular centres 

 of the ear have an important function with relation to the 

 sense of position and equilibration ; and the cerebellum is 

 expanded in size in mammals as the centre of muscular 

 movements, of muscle sense and activities in general. 



The medulla oblongata is formed of tracts continuous 

 with those of the spinal cord, and is the seat of centres 

 concerned with involuntary movements. 



The fourth ventricle is included between the medulla and 

 the cerebellum. It is surrounded by the thick walls of the 

 medulla and has a thin roof below the cerebellum. 



The brain is covered by the pia mater, which leads blood- 

 vessels into the walls. Vessels are carried in at the velum 

 interpositum and the fourth ventricle, forming in each case a 

 choroid plexus. The skull is lined by the dura mater carrying 

 the meningeal vessels, and between the two layers is the arach- 

 noid space containing a watery fluid and lined by a delicate 

 inner and outer membrane and extending the length of the 

 brain and spinal cord. 



The cranial nerves are described as twelve in number. The 

 olfactory is associated, however, with a nervus terminalis. 



I. olfactory ; sensory, olfactory lobe, numerous branches 

 passing through the cribriform plate to reach the sensory cells 

 of the nasal epithelium. 



II. optic ; sensory, optic lobes, thalami and cerebrum. 

 The fibres forming the optic tract decussate in the optic chiasma, 

 and the optic nerves pass into the respective orbits through 

 the optic foramen, enter the eye and expand into the retina. 



III. oculomotor ; motor, floor of mesencephalon, leaves the 

 skull by the foramen lacerum anterius, and supplies the superior, 

 internal, inferior recti, and the inferior oblique muscles of the 

 eye. 



IV. trochlear ; motor, decussates on the roof of the brain 

 just behind the optic lobes, and each nerve leaves by the same 

 foramen as the previous one, supplies the superior oblique 

 muscle of the eye. 



V. trigeminal ; mixed, anterior end of medulla, originates 



