816 MAMMALIA. 



(fig. 411, c c); and coexistent with these thepons Varolii (d) makes its 

 appearance, embracing the medulla oblongata and uniting the opposite 

 sides of the cerebellum. 



(2403.) The structure of the spinal cord and the origins of the spinal 

 nerves throughout all the Mammalia are precisely similar, and exactly 

 correspond with what occurs in the human body; neither does the ana- 

 tomical distribution of the individual nerves derived from this source 

 require any special notice, since, generally speaking, it differs in no 

 important particular from the arrangement with which every anatomist 

 is familiar. 



(2404.) The sense of touch in Mammalia is diffused over the whole 

 surface of the body, its perfection in different parts being of course 

 influenced by the nature of the integument, and the number of sentient 

 nerves appropriated to any given region. All the nerves derived from 

 the sensitive tract of the spinal medulla, and the three divisions of the 

 fifth pair of encephalic nerves, are equally susceptible of tactile im- 

 pressions ; so that, in a class so extensively distributed as that before 

 us, we need not be surprised to find a special apparatus of touch deve- 

 loped in very different and remote parts adapted to particular exigen- 

 cies. Thus the whiskers of the Seal and of nocturnal Carnivora, the 

 lips of the Horse, the trunk of the Elephant, the hands of Man, the 

 hind feet of the Quadrumana, and even the extremity of the tail where 

 that organ is prehensile, are all in turn made available as tactile instru- 

 ments, and exercise the sense in question with the utmost delicacy. 



(2405.) In the Bats, where the sense of vision becomes inadequate 

 to guide them through the dark recesses where they lurk, that of touch 

 assumes its utmost development, and every part of the body that could 

 by possibility be furnished with it has been abundantly provided for in 

 this respect. Not only is the broad expanse of the wing acutely sensible, 

 but the very ears have been converted into delicate feelers ; nay, from 

 the tip of the nose in some species, membranes of equal sensibility have 

 been largely developed ; so that the Bats, as was ascertained by Spallan- 

 zani, even when deprived of sight and hearing, will fly fearlessly along, 

 and avoid every obstacle with wonderful precision, guided apparently 

 by the sense of touch alone. 



(2406.) The sympathetic system of the Mammifera differs in no im- 

 portant particular from the human, the arrangement of the ganglia and 

 the distribution of the plexuses being in all respects the same. 



(2407.) In the conformation of the genito-urinary apparatus in 

 Mammalia the physiologist will find many circumstances of extreme 

 interest. 



(2408.) Even in Birds, as the reader will remember, the secretions 

 of the testes and of the kidneys were both poured into the common 

 cavity of the cloaca, and discharged through the anal orifice. No 

 bladder was provided for the reception of the urine; and a simple, 



