166 EMBRYOLOGY 



hormones. The nervous portion, or posterior lobe, secretes hormones which 

 control blood pressure and the contraction of the smooth muscle of the 

 uterine wall. This dual embryonic origin results in a gland with dual 

 function. Similarly, the adrenal gland has a dual embryonic origin and it too 

 has a dual function. Thus studies of the development of structures often clarify 

 their function in the adult. 



The epiphysis arises as an evagination of the dorsal wall and becomes the 

 pineal gland in the adult. The exact function of this gland is not clear. Some 

 endocrinologists believe that it may have some possible function during sex 

 differentiation. 



There are still the lateral walls and cavity of the diencephalon to be 

 considered. The lateral walls of the diencephalon form the thalamus, which 

 establishes connections with the cerebral hemispheres. The cavity of the 

 diencephalon becomes the third ventricle of the brain. 



The mesencephalon, or midbrain, is sometimes considered as a third 

 division of the forebrain. It gives rise to the optic lobes, in which some of the 

 fibers of the optic nerve connect with other neurons. The cavity of the 

 mesencephalon becomes constricted to a narrow canal connecting the third 

 and fourth ventricles of the brain. 



The metencephalon forms the cerebellum from its dorsal wall. The func- 

 tion of this structure is the control of postural reflexes. The floor of the 

 metencephalon carries fiber tracts. 



The myelencephalon forms the medulla, which contains fiber tracts 

 running into the spinal cord. It also develops brain centers which consist of 

 cell bodies which regulate respiration and circulation. The cavity of the 

 myelencephalon forms the fourth ventricle, which is continuous with the 

 central canal of the spinal cord. 



In some places the roof of the brain becomes very thin, and such an area 

 develops into a choroid plexus. A choroid plexus grows into the cavity of 

 the brain, acquires a good blood supply, and functions to supply the brain 

 with nutriments. 



The cranial nerves, like the spinal nerves, originate from two sources. 

 The motor neurons grow out from the brain and supply muscles or glands as 

 effectors. The sensory nerve fibers originate from the ganglia or, as in the 

 case of the olfactory and optic nerves, from neuroblasts within the sense organ 

 itself. A detailed description of the structure and function of these nerves 

 may be found in textbooks on comparative anatomy or vertebrate anatomy. 



