SKIN AND ITS APPENDAGES. 541 



enter the lachrymal sac, the superior, being the longer, a little 

 behind the inferior. 



The Lachrymal sac is a small reservoir lodged in a fossa in 

 the lachrj'mal bone, and is the origin of the lachrymal duct. 



The Lachrymal or Nasal duct (ductus ad nasum) is a long 

 membranous canal, which begins at the sac, passes through a 

 conduit in the bone, and terminates on the internal face of the 

 nasal opening, on the skin, near to and a little above the inferior 

 commissure of the ala ; it sometimes terminates in two openings. 

 It is lined by a continuation of the conjunctiva, which also lines 

 the lachrymal canals and sac. In some animals it is partly lined 

 by an inflexion of the mucous membrane of the nostril. 



The Ocular sheath is a dense fibrous membrane lining the 

 orbit and enclosing its contents. It is strongest supero-externally, 

 where the cavity is deficient in osseous boundary. It has been 

 said that this membrane is continuous with the dura mater, to 

 which it bears resemblance in structure. 



The senses of Taste and Smell are associated respectively with 

 the tongue and nasal chambers, which have previously been con- 

 sidered. 



Skin and its Appendages. 



The skin and its appendages constitute the exoskeleton or 

 external casing of the body. The skin itself consists of two 

 parts, the dermis, cutis vera, or corium, which forms the deep 

 layer, and the epidermis or cuticle, superficially placed. Its 

 chief appendages are glands, and the epithelial modifications, 

 hair, horn, and hoof. 



The Epidermis, cuticle, or scarf skin, is an epithelial structure, 

 forming a protective covering to the corium. It varies in thick- 

 ness, is quite insensitive and non-vascular, and consists of agglu- 

 tinated cells which are formed on the surface of the true skin • 

 they contain nuclei, vary in form, the deep layers being columnar, 

 those above rounded, those on the surface flat, being finally cast 

 off as dry, desquamating scales. They also change in physical and 

 chemical constitution, the deep ones being soft and opaque, the 

 superficial dry, horn}'-, and soluble in acetic acid. Their fluid 

 portion when pressed out serves to cement them together. The 

 epidermis is divided into a firm and transparent superficial and a 

 deep soft layer. The latter is the rete mucosum, in whose cells 



