184 



HUMAN EMBRYOLOGY AND MORPHOLOGY. 



(6) The sclerotic. — -This is the outer covering or tunic of nieso- 

 blast. It is continuous in front with the cornea ; behind with the 

 optic nerve sheath. In some vertebrates, but not in mammals, 

 plates of bone are developed in the anterior half of the sclerotic. 



The Tapetum lucidum is absent in the human and primate eye. 

 It gives the metallic lustre seen on the retinal surface of the eye 

 of the ox, and is formed by a layer of fine fibres which are 

 developed on the retinal surface of the choroid. 



(7) The Capsule of Tenon, the bursa or connective-tissue socket 

 of the eyeball, and is developed in the mesoblast surrounding the 

 eyeball. A lymph space separates it from the sclerotic, which 

 is but slightly marked until after birth. The choanoid muscle 

 (retractor bulbi or Midler's muscle) which surrounds the posterior 

 part of the eyeball as a muscular hood in mammals and verte- 

 brates generally, has disappeared in man and the higher primates. 

 Probably its fibrous remnant helps to form the capsule of Tenon. 



Formation of the Orbit (Fig. 150). — The orbit is formed 

 (1) above by the capsule of the fore-brain in which the frontal 



nasal 

 {lat. nas. proc.% 



asc. proc. max. 

 (lat nas. proc). 



ethmoid 

 (lat. nas. proc.) 



-malar {max. process) 



\~jy-sup. max. (max. process) 

 position of nasal duct. 



Via. 150.— The Origin of the Bones entering into Formation of the Orbit. 



bone is developed ; (2) externally and below by the maxillary 

 process (Fig. 1). In the maxillary process the malar bone and 

 superior maxilla (except the ascending nasal process) are 



