EYES. LACRYALA.L GLANDS. 407 



(sinus frontales, sphenoidales, maxillares). In the Cetacea the 

 external nostrils are sometimes united to form a single opening. 

 The nasal openings are often placed at the end of a prolongation 

 of the face, which is sometimes greatly developed, as in the 

 trunk of the elephant. In the aquatic mammals they can be 

 closed by muscles or a valvular apparatus. Jacohsori's organ is 

 present in many mammals (Marsupialia, Edentata, Insectivora, 

 Carnivora, Ungulata), in the form of two tubes placed ventrally 

 to the septum nasi and receiving a branch of the olfactory nerve. 

 They generally join Stenson's duct and so open into the mouth. 



The eyes present the normal vertebrate structure. Their 

 most noticeable features are perhaps the presence of retinal 

 bloodvessels and the absence of any structure corresponding to 

 the pecten. They are always much reduced and may be quite 

 vestigial in burrowing animals. In some rodents and insecti- 

 vores {Tolpa, Chrysochloris) and in the marsupial Notoryctes 

 they are hidden beneath the skin, and in the freshwater cetacean 

 Platanista they are very imperfectlj'^ developed. Both upper 

 and lower lids, which are usually covered with hair, are present, 

 and in addition there is a third transparent eyelid at the inner 

 angle of the eye, the nictitating membrane. The nictitating 

 membrane is absent in Primates where it is represented by the 

 plica semilunaris, and in the Cetacea. The cornea is fairly con- 

 vex in most forms, but flattened in the Cetacea. There is a 

 tapetum, which reflects the light, in the choroid coat in many 

 mammals (Carnivora, Ungulata, etc.). 



Both harderian and lacrymal glands are present in most 

 Mammalia (absent or reduced in the whales, and reduced 

 in the Pinnipedia). The lacrymal gland has several openings 

 on the conjunctival surface beneath ths upper lid towards the 

 posterior (outer) side of the eye ; the harderian gland (which 

 lies at the inner side of the eyeball and mainly on its lower 

 surface, opens in connection with the nictitating membrane at 

 the inner angle. The harderian gland is cbsent in Primates. 

 The nasal ducts open on the puncta lacrymalia of which there are 

 two at the inner angle of the eye one above and one below the 

 caruncula lacrymalis. These two ducts join to form the nasal 

 duct which opens into the nasal passage. The meibomian 

 glands are sebaceous glands placed in the lids beneath the con- 

 junctiva and opening on the free edges of the latter. 



z.— II ^ ^ 



