THE HEAD AND NECK. 145 



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 " gut- 

 tural pouches " — also diverticula of the respiratory 

 passages — large cavities containing only air, one on 

 each side, situated at the base of the skull behind 

 the pharynx, and connected with the Eustachian 

 tubes (the canals which convey air to the internal 

 chamber of the ear) and which in the most approved 

 works on veterinary anatomy are said to be "found 

 only in solipeds." Exactly similar pouches exist in 

 the tapir, but I am not aware whether they have as 

 yet been looked for in the rhinoceros. They have 

 been supposed to have some use in connection with 

 the function of hearing ; but it is possible they may 

 rather be classed with the numerous large air sinuses 

 found within the bones of the head of the horse, in 

 common with most other mammals, the object of 

 which is evidently to give increased volume without 

 increased weight, and thus to furnish wide surface 

 for the attachment of muscles and for the support 

 and protection of various organs situated within the 

 head. 



It has been pointed out that in the artificial con- 

 ditions under which some domestic horses live, these 



