THE IIKAD AND NECK 



135 



these three very distinct forms, and in, as far as is known, 

 no other mammal, is a strong corroboration of the 

 view, formed upon other evidence, of their close affinity 

 and common descent. 



In the ass, the pouch is said to be deeper than in 

 the horse, and areolated at the blind extremity ; but 

 detailed comparative observations upon its condition 

 in the different species of existing Equidce and upon its 

 development in the horse are almost entirely wanting, 

 and would well repay the trouble the investigation 

 would cost. The nearest analogue in other orders of 

 mammals is perhaps the singular pouch developed from 

 the upper part of the commencement of the nasal 

 passage of the 'bladder-nosed' seal (Gystophora cristata), 

 which the animal has the power of inflating with air 

 when excited. The analogy is, however, by no means 

 close, as in the seal the pouch is only found in the male, 

 and not even in the young of that sex. If the sac in 

 the horse is the remnant of some organ which formerly 

 played a more important part in the economy of the 

 race, we should expect to find it proportionately larger 

 in the younger individuals of the existing species than 

 in the adults. 



Guttural Pouches 



Other equally mysterious structures are the 4 guttural 

 pouches — 7 also diverticula of the respiratory passages — 



