288 THE CAT. [CHAP. jx. 



Thus the maxillo-turbinals do not minister to smell, but serve, as 

 it were, to strain and also to warm the air at first received within 

 the nostrils, and which is subsequently diffused into the truly 

 olfactory upper chamber. 



In connection with the nasal cavity a peculiar structure, called 

 the organ of Jacolson, may be noticed. This is a small tubular sac 

 which is placed on each side of the median septum upon the nasal 

 surface of the palatine plates of the premaxilla and maxilla. Pos- 

 teriorly it enus blindly, but anteriorly it opens by the incisive 

 foramen into the cavity of the mouth, the mucous membrane of 

 which is continued into it. It receives nerves from the olfactory 

 bulb, which enter into its hinder end and which terminate in struc- 

 tures analogous to the filamentary terminations of the olfactory 

 nerves of the sensory part of the nasal cavity. Though these organs 

 open into the mouth, and not into the nasal fossa?, yet they have an 

 essential relation to the latter, as will appear in the next chapter. 



25. The ORGAN OF SIGHT, the eye, consists, as has been already 

 said, of a globe of more or less soft tissues, into the outer surface of 

 which muscles are inserted the whole being protected and enclosed, 

 except in front, by dense fascia and muscle, or else by the osseous 

 plates which form the imperfectly closed bony socket or orbit of the 

 eye. Into this ball, moreover, as has already been stated, the optic 

 nerve enters, passing through the optic foramen to its posterior part. 



In front, where the bony protection ceases, the ball is protected 

 by an extension of skin above and below, forming the eyelids or 

 palpebrce. These have their outer surfaces covered by the external 

 skin, while each is lined internally with mucous membrane, and the 

 mucous membranes of both the upper and lower eyelid of each eye 

 are continued one into the other by that transparent membrane 

 which covers the front of the eyeball, and is called the conjunctiva. 

 Internally they are strengthened by a strong fibrous membrane. 



The upper eyelid is raised by the special muscle called the 

 " levator palpebrae," which is supplied by a branch of the oculo- 

 motor nerve. A circular sphincter muscle (the orbicularis palpe- 

 brarum) extends through both eyelids, and, by its contraction, closes 

 them. There is no special depressor of the lower eyelid. The 

 point on each side where the eyelids unite is termed the angle, or 

 canthus, of the eye. At its inner canthus are two minute apertures 

 (to receive the lachrymal secretion) called puncta lachrymalia. 

 There is also a large fold of membrane, or third eyelid the plica 

 semilanaris or membrana nictitans which rises from the bottom and 

 inner angle of the orbit and rests upon the eyeball. It has a 

 cartilage at its margin which strengthens it (Fig. 130, MN). 



Immediately beneath the thin delicate outer skin of the eyelids, 

 and adherent to it, are the fibres of the orbicularis muscle, and 

 beneath these are the fibrous membranes of each eyelid (defining 

 its shape and giving it firmness), and then the levator palpebrai 

 muscle. On the inner surface of each lid are certain sebaceous 

 follicles, or tubes, termed Meibomian glands, which extend vertically 



