186 



HUMAM EMBRYOLOGY AND MORPHOLOGY. 



lachrymal canaliculi and sac and nasal duct are formed out of" 

 solid epithelial cords enclosed between the maxillary and lateral 

 nasal processes (Fig. 151). 



plica semilunaris 

 canaliculus 



lach. sac 

 nas. duct 

 value 



Fig. 151.— Diagram of the Plica Semilunaris and Lachrymal Canaliculi. 



The Orbital Muscles. — We have already seen that the head 

 is composed of nine segments, at least four of these being 

 occipital ; also, that each segment gives rise to a muscle plate. 

 The muscle plate of the first head segment forms the muscles 



inf. canal. 



to sup. rectus, etc. 



Fig. 152.- 



-for. Monro 

 3rd uent. 

 corp. quad, 

 motor nucleus 

 1st segment 

 of 2nd segment 



4th uentricle 

 of 3rd segment 



-Diagram of the Motor Nerves of the Muscles of the Eye derived from the 

 1st, 2nd, and 3rd Cephalic Segments. 



supplied by the third cranial nerve — the motor nerve, of the first 

 cephalic segment. The mesencephalon (crura cerebri) contains 

 the corresponding segment of the neural canal. The ciliary 

 muscle and sphincter of the iris also belong to this segmentand 



