Y2 THE ANATOMY OF VERTEBRATED ANIMALS. 



and, in Myxine^ the two pneumogastrics unite upon the intes- 

 tine, 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' 

 alreadj'- 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 integu- 

 ment. 



The Olfactory A2)pa7'atus acquires no higher complication 

 than this, being either a single sac {Amjyhioxiis (?) Marsipo- 

 branchii)^ or, more commonly, two, the surfaces of which are 

 increased by plaiting, or by the development of turbinal carti- 

 lages, or bones, from the lateral portions of the ethmoid. 

 Upon these, nervous filaments arising from the olfactory lobe 

 of the brain are distributed. The cavities of the olfactory 

 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 frequently connected with, and pours its secretion into, 

 each olfactory chamber. 



The foramina incisiva, left between the premaxillaries 

 and the palatine plates of the maxillaries in Mainnialia^ are 

 sometimes closed by the mucous membranes of the nasal and 

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

 the canals qf'Stensoji^ and place these two cavities in com- 

 munication. Glandular diverticula of the mucous membrane, 

 supplied with nervous filaments from both the olfactory and 

 the fifth pair, may open into these canals. Thej^ are called, 

 after their discoverer, the " organs of Jacohsony 



The Eye is formed by the coalescence of two sets of struct- 

 ures, one furnished by involution of the integument, the other 

 by an outgrowth of the brain. 



The opening of the integumentary depression which is pri- 

 marily formed on each side of the head in the ocular region 

 becomes closed, and a shut sac is the result. The outer wall 

 of this sac becomes the transparent cornea of the eye ; the 

 epidermis of its floor thickens, and is metamorphosed into the 

 crystalline lens / the cavitj'' fills with the aqueous humor. A 

 vascular and muscular ingrowth taking place round the cir- 

 cumference of the sac, and, dividing its cavity into two seg- 

 ments, gives rise to the iris. The integument around the cor 



