2io THE ORGANS OF SPECIAL SENSE 



is partially or completely inhibited in toxic conditions. It will 

 be found, therefore, that children who are seriously ill rarely 

 shed tears. 



The puncta lacrimalia are normally in apposition with the 

 ocular conjunctiva, and only under these conditions can the 

 secretion enter the lacrimal ducts and so drain away through 

 the lacrimal sac and naso-lacrimal duct into the nose. In 

 paralysis of the orbicularis oculi (p. 82), the puncta fall away 

 from the surface of the eye and the lacrimal secretion, being 

 unable to enter the ducts, overflows on to the cheek, constitut- 

 ing the condition of epiphora. The same condition will arise 

 if the punctum is too narrow, either congenitally or following 

 inflammatory conditions, or if there is any obstruction in the 

 naso-lacrimal duct. 



2. THE EYELIDS. The skin of the upper eyelid is supplied 

 by the supra-trochlear, the supra-orbital and the lacrimal 

 nerves (ophthalmic division of V.), while that of the lower eye- 

 lid is supplied entirely by the infra-orbital branch of the 

 maxillary division (p. 69). Under the skin lies the orbicularis 

 oculi (palpebrarum) (p. 82) and, at the margins of the eyelids, 

 the unstriated ciliary bundle is found. It derives its nerve- 

 supply, not from the facial like the rest of the muscle, but 

 from the sympathetic. Paralysis of these involuntary fibres 

 gives rise to a variety of ptosis which has been termed pseudo- 

 ptosis (p. 189). 



A plate of condensed fibrous tissue lies in each lid deep to 

 the fibres of the orbicularis oculi. These are termed the 

 superior and inferior tarsi, and the former is much the larger 

 of the two. A thin ligamentous sheet extends from the 

 margins of the bony orbital aperture to blend with the tarsi. 

 Though not particularly strong, it is sufficient to influence the 

 course of intra-orbital haemorrhage, which is guided downwards 

 behind the conjunctiva (Fig. 82). 



The deep surfaces of both eyelids are covered by the con- 

 junctival mucous membrane. From the muco-cutaneous 

 junction the conjunctiva passes over the lid and it is then 



