THE EYE. 707 



child expand, the pulmonary arteries increase in size, and the 

 ductus arteriosus dwindles in a corresponding degree. 



The liver, which in the foetus is of relatively greater size than 

 in the adult, receives much blood coming from the placenta to 

 the heart, and is thought to contribute to it several essential con- 

 stituents. 



The head and brain, which are largely developed in the fcetus, 

 receive well-aerated blood, namely, the placental blood, which 

 has passed through the liver, and, in the inferior vena cava, is 

 mixed with blood coming from the lower limbs. The rest of the 

 fcetus receives blood that is less well aerated, as it is mixed with 

 that which is returned from the head and neck to the right side 

 of the heart, and which is sent through the ductus arteriosus to 

 join the general blood current in the aorta going to the viscera 

 and lower extremities. 



DEVELOPMENT OF THE EYE. 



The optic vesicles arise from the anterior cerebral vesicle at a 

 very early period, and their cavities are continuous with that of 

 the fore-brain. With the development of the rudimentary cerebral 

 hemispheres the optic vesicles become displaced downward, and 

 their cavities open into the junction of the cavities of the cerebral 

 hemispheres and that of the thalamencephalon, which becomes 

 the third ventricle. Later, the optic vesicles open directly into 

 the third ventricle, and finally are displaced backward, and 

 come into connection with the mid-brain. 



The optic vesicles are at first hollow prolongations, which con- 

 sist of an anterior dilated portion, forming the primary optic 

 vesicle, and a posterior tubular portion or stalk joining the 

 vesicle to the fore-brain. This stalk forms the optic nerve. 



As each vesicle grows forward toward the epiblast covering 

 the head of the embryo, the epiblastic cells at the spot overlying 

 the vesicle become thickened, and an involution of the epiblast 

 takes place toward the optic vesicle, and indents the latter, ap- 

 proximating its anterior to its posterior wall. 



By this means the anterior and posterior walls of the primary 

 optic vesicle come into close contact, and the cavity of the vesicle 



