CHAP, vii THE MEDULLA OBLONGATA 381 



cerebral hemispheres. Each hemisphere consists of the cortex, and 

 the caudate and lenticular nuclei which constitute the corpus 

 stria turn. 



(&) The 'tween-brain or thalamencephalon. In the embryo this 

 is represented by the 2nd cerebral vesicle, the lateral walls of which 

 thicken and form the optic thalami. The third ventricle lying 

 between the thalami represents the 1st primary cerebral vesicle. 



(c) The mid-brain or mesencephalon is formed by the thicken- 

 ing of the walls of the 3rd embryonic vesicle. Its ventral part 

 forms the cerebral peduncles, the dorsal part the optic lobes or 

 corpora bigemina of the lower vertebrates, the corpora quadrigemina 

 of mammals. The aqueduct of Sylvius by which the third and 

 fourth ventricles communicate is the remains of the embryonic 

 mesencephalic vesicle. 



(d) The hind-brain or metencephalon develops from the 4th 

 secondary vesicle. The thickening of the ventral wall gives rise 

 to the pons Varolii, of the dorsal walls to the cerebellum. The 

 fourth ventricle or sinus rhomboidalis is the remains of the 

 embryonic vesicle. 



(e) The medulla oblongata or myelencephalon is derived from 

 the 5th secondary vesicle, the ventral portion of which enlarges to 

 form the bulb or medulla oblongata, while the dorsal part remains 

 a simple epithelial layer adherent to the pia mater which covers 

 the sinus rhomboidalis. 



In order to form an idea of the very unequal development of 

 the five embryonic segments in the brain of the adult, the corre- 

 sponding parts of Figs. 199 and 200 should be compared. The first 

 represents the brain of a human embryo, at two and a half months ; 

 the second, the adult brain. It will be seen that in the foetus the 

 thalamencephalon and mesencephalon are relatively very large, 

 while in the adult the cerebrum, and after it the cerebellum, are 

 largest, and the corpora quadrigemina are relatively small. 



Anatomical text-books should be consulted for the external form 

 and internal structure of the brain : here we must confine our- 

 selves to such anatomical details as are necessary to the study of 

 its physiology. 



The spinal bulb, which is the subject of the present chapter, is 

 the intracranial prolongation of the spinal cord, hence the name 

 medulla oblongata. Owing to its vital importance, and the 

 multiplicity of its functions, it is quite one of the most important 

 parts of the nervous system. The complexity of its structure 

 indicates the complexity of its functions. 



It is conical in form, with the base above, at its junction with 

 the pons, and a truncated apex below, continuous with the spinal 

 cord. As shown by Fig. 201, the cerebral nerves from the hypo- 

 glossal (12th) to the abducent (6th) issue from the ventral and 

 lateral surfaces of the bull >. 



