304 STRUCTURE OF THE VERTEBRATES 



bones, are ordinarily not covered with olfactory epithelium, 

 but with serous cells which tend to warm and moisten the air. 

 The posterior ethnio-tiirbinals push in between those from the 

 dermal bones, and carry the olfactory receptors. 



The external nares are separated by a cartilage projecting 

 anteriorly from the mesethmoid. Laterally the nares are enclosed 

 by fleshy bands of tissue, which may be prolonged into a snout 

 or, in the tapir and elephant, a greatly elongated proboscis. 



Water living mammals tend to have a poorly developed 

 sense of smell. The toothed whales lack even the olfactory nerves 

 and the foramina of the cribriform plate. The primates also have 

 a small sensory area, and in comparison with the rodents, 

 carnivores, and most other orders of mammals an inefficient 

 sense of smell. 



Development in the Human. In the embryo of five weeks' 

 development the mouth is wide, the nose is broad and flat, and 

 the nostrils are widely separated and connected with the oral 

 cavity by grooves. The median nasal region rapidly becomes 

 elevated as the lateral masses of tissue grow medially, forcing 

 the nares closer together until they are separated by a narrow 

 septum and the passages look downward. The upper lip is 

 formed by this median growth, completely cutting the nares 

 from the mouth. The line of fusion is left in the adult as the 

 median line of the upper lip. A failure of these tissues to coalesce 

 causes hare lip, a common defect of the human. Less frequently 

 the maxillary bones fail to meet in the mid-line and cleft palate 

 results. 



D. The Eyes 



The vertebrate eye differs from that of the invertebrate in 

 the inversion of layers of the retinal cells. That is, the eye 

 receptors of the vertebrate lie nearest the supporting cover, 

 with the association neurones between the sensory layer and 

 the source of light. The invertebrates have the direct method 

 of receiving light rays directly against the sensory cells, the 

 association neurones being deeper and next the supporting layers. 

 But, after the vertebrate eye developed, there has been little 



