332 ZOOLOGY FOR MEDICAL STUDENTS chap. 



The various subdivisions of the brain as enumerated above are 

 commonly grouped into either three or two primary divisions. The 

 older division into three — Fore-brain (=Thalamencephalon or Primary 

 fore-brain + Hemispheres or Secondary fore-brain), Mid-brain ( = Mesen- 

 cephalon) and Hind-brain (= Cerebellum -i- Medulla oblongata) — rests 

 mainly on the fact that in some of the vertebrates of which the develop- 

 ment has long been familiar, e.g. the Birds, the brain becomes at a very 

 early period of its development marked ofi by slight constrictions into 

 portions lying one behind the other which give rise to fore-, mid- and 

 hind-brain respectively. The more modern custom of regarding the 

 primary subdivisions of the brain as two in number — Cerebrum and 

 Rhombencephalon — is based especially on the work of anatomists but 

 it is also supported by the embryology of some of the more primitive 

 vertebrates in which the Cerebrum becomes marked off from the 

 Rhombencephalon long before it shows a subdivision into fore -brain 

 and mid-brain. 



From the brain there pass off ten pairs of cranial nerves which are 

 found with hardly any modification through the series of vertebrates 

 right up to the highest. They are as follows : 

 L Olfactory. 

 IL Optic. 

 in. Oculomotor. 

 IV. Pathetic. 

 V. Trigeminal : 



1. Ophthalmic. 



2. Maxillary. 



3. Mandibular. 

 VI. Abducent. 



VII. Facial : 



1. Ophthalmic. 



2. Buccal. 



3. Palatine. 



4. Hyomandibular. 

 VIII. Auditory. 



IX. Glosso-pharyngeal. 

 X. Vagus : 



1. Visceral. 



2. Lateral. 



Olfactory. — The First Cranial nerve consists of nerve-fibres passmg 

 from the organ of smell to the ganglion-cells of the olfactory bulb. It is 

 a purely sensory nerve, devoted entirely to the sense of smell. In the 



