GENERATION AND DEVELOPMENT. 287 



mingle in this cavity of the heart, and passing into the right ventricle, 

 go by way of the pulmonary artery to the lungs, and through these, after 

 purification, to the left auricle and ventricle, to be distributed over the 

 body. (See Chapter on Circulation.) 



FIG. 444. Diagram of the Foetal Circulation. 



DEVELOPMENT OF THE NERVOUS SYSTEM. 



Nerves. All the spinal nerves are derived from the mesoblast; also 

 all the cranial nerves, except the optic and olfactory, which are out- 

 growths of the anterior cerebral vesicles. From the same middle layer of 

 the embryo are also derived the ganglia connected with these nerves, and 

 the whole sympathetic system of nerves and ganglia. 



Spinal Cord. Both the brain and spinal cord have a different origin 



