DIGESTIVE SYSTEM 



739 



ends of the horns are fastened to the bottom of the sheaths so that the 

 sheath is turned inside out when they are withdrawn. 



The tongue develops between the hyoid and mandibular arches (Fig. 

 432). The hyoid often extends into and supports the tongue. Conse- 

 quently, the organ itself cannot be moved unless its supporting skeleton 

 is likewise moved. The tongue is a sensory organ but can be used as an 

 organ of touch and taste. There are little elevations (Fig. 433) known 

 as papillae, in many if not most animal tongues. Some of these are 



mth 



Fig. 432. 



Two stages in the development of the tongue and pharyngeal 

 floor of man. c, copula (basihyal element) ; cs, cervical sinus; ep, 

 epiglottis ; g, glottis ; h, hyoid arch ; md, mandibular arch ; mth, 

 median anlage of thyroid gland ; t, tuberculum impar ; tg, tongue. 

 (P'rom Kingsley after His.) 



Fig. 433. 



Papillary surfaces of the human 

 tongue showing fauces and tonsils. 

 1, 1, circumvallate papillae, in front 

 of 2, the foramen caecum ; 3, fungi- 

 form papillae ; 4, filiform and con- 

 ical papillae ; 5, transverse and 

 oblique rugae; 6, mucous glands at 

 the base of the tongue and in the 

 fauces ; 7, tonsils ; 8, part of the 

 epiglottis ; 9, median glosso-epi- 

 glottidean fold (frenum epiglot- 

 tis). (From Hill after Sappey.) 



sensory while others have become hardened and serve as rasping organs. 

 In the cyclostomes the tongue is thick and fleshy and is supported 

 by a cartilaginous skeleton. The muscles which throw out the tongue 

 are called protractor muscles and those which draw the tongue back to 

 its normal position are known as retractors. These muscles are devel- 

 oped from the postotic myotomes and their nerve supply comes from the 

 hypo-glossal nerve. In the myxinoids the terminal end of the tongue 

 possesses epidermal teeth which thus form a boring organ by which these 

 animals obtain entrance into their prey. In the lampreys the surface 

 has a rasping organ and also forms part of the sucking apparatus. In 

 the amphibians, there are a few anura (aglossa), in which the tongue is 

 practically absent, but in most of them the tongue actually contains in- 

 trinsic muscles supplied by the. hypo-glossal nerve and in such case the 

 tongue can be moved quite readily. The tongue of amphibians is made 

 up of a small basal portion quite similar to that of the fish, but to this 



