$72 COMPARATIVE ANATOMY 



II. Conchae of the primitive sensory epithelium : 



1. Arising from the lateral wall (conchae laterales). 



A. Anterior: 



(a) Primary, ventral : concha of reptiles ; middle concha 



of birds ; maxillo-turbinals of mammals. 



(b) Secondary, dorsal: Upper or posterior of birds; naso- 



turbinals of mammals (Pseudoconch of crocodiles). 



B. Arising from the posterior part: conchae obtectae of 



mammals. 



2. Arising from the primitively median wall : ethmo-turbinals 

 of mammals, numbered from in front backward. 



Jacobson's organ (Fig. 339), also called the vomero-nasal organ, 

 can be seen in the embryo of most mammals as a pocket on the lower 

 middle side of each nasal cavity. It opens near the duct from Stenson's 

 gland in rodents and in man, while in other mammals its duct is appar- 

 ently cut off from the nasal cavity and opens into the naso-palatal canal. 

 Its middle wall is covered with a sensory epithelium to which branches 

 of the olfactory nerve extend. The organ degenerates in adult primates. 



There are two kinds of glands in the nasal cavity, known as Bow- 

 man's glands, which are the smaller, and Stenson's gland, which lies in 

 the lateral ventral wall and opens into the vestibule. There are usually 

 sinuses in the bones of the skull connected with the nasal cavity by 

 various openings. Figure 489 shows several of the principal sinuses 

 in the bones, such as the maxillary, frontal, and sphenoidal. 



All mammals have an external fleshy nose supported by nasal bones 

 and cartilage, but in swine the fleshy portion forms a proboscis of con- 

 siderable size, and in the elephant this fleshy proboscis extends tremen- 

 dously to form the trunk. 



It is interesting that while most mammals have a well developed 

 sense of smell, in seals, whale-bone whales, and primates, it is not very 

 great, while it is practically absent in the toothed-whales. Often the 

 olfactory nerve disappears entirely. 



THE EYE (Figs. 18, 289, 338) 



A review of the embryology of the eye in both frog and chick will 

 make clear how the eye cup, vesicle, stalk, and lens are formed. 



A detailed description of the eye must be sought in text-books of 

 histology. Here we can give but a general outline so as to make intel- 

 ligible the dissection and comparison of the eye in the various groups of 

 animals studied. The adult eyeball is made up of three tunics, tabulated 

 as follows : 



I. Tunica Externa. 



1. Sclera. 



2. Cornea. i 



