78 THE ANATOMY OP VEETEBRATED ANIMALS. 



when complete, from all the cranial nerves except those of 

 the special senses of heai-ing, sight, and smell — the Vidian 

 nerves constituting the anterior terminations of the sympa- 

 thetic cords. At the points of communication ganglia are 

 developed, and the nerves which emerge from these ganglia 

 are distributed to the muscles of the heart and vessels, and 

 to those of the viscera. These peripheral nerves of the 

 sympathetic system frequently present small ganglionic 

 enlargements. 



In the Marsipobranchii, the place of the sympathetic 

 appears to be taken, to a great extent, by the pneumogastric ; 

 and, in Myxine, the two pneumogastrics unite iipon the in- 

 testine, and follow it, as a single trunk, to the anus. 



The Sensory Organs. — The organs of the three higher 

 senses— Smell, Sight, and Hearing — are situated, as has 

 been already described, in pairs, upon each side of the skull, 

 in all vertebrate animals except the lowest fishes ; and, in 

 their earliest condition, they are alike involutions of the 

 integument. 



The Olfactory Apparatus acquires no higher complication 

 than this, being either a single sac (Ampliioxus (?) 3Iarsipo- 

 hranchii), or, more commonly, two, the sui-faces of which 

 are increased by plaiting, or by the development of turbinal 

 cartilages, or bones, from the lateral poi-tions of the eth- 

 moid. TJj)on these, nervous filaments arising from the olfac- 

 tory lobe of the brain are distributed. The cavities of the 

 oKactory sacs may be placed in communication with that 

 of the mouth by the nasal passages ; or, as in the great 

 majority of fishes, they may have only an external aperture, 

 or apertures. 



In Reptiles, Birds, and Mammals, a peculiar nasal gland 

 is freqviently connected with, and pours its secretion into, 

 each olfactory chamber. 



The foramina incisiva, left between the premaxiUaries and 

 the palatine plates of the maxillaries in Mammalia, are 

 sometimes closed by the mucous membranes of the nasal and 

 oral cavities, and sometimes not. In the latter case they 



