GENERAL INTERNAL ORGANIZATION 



43 



mouth cavity proper at its posterior end. This latter relationship 

 may be seen by looking into the widely opened mouth of another 

 person, or into one's own mouth by means of a mirror. The 

 Eustachian tubes (Fig. 23), which lead from the mouth cavity of 

 the frog to the cavity of the ear just inside the eardrum, have a 

 similar relationship in man, save that they communicate at their 

 inner ends with the nasal cavity (Fig. 24). Again, the long axis of 

 the frog's mouth is in Hne with that of the body, while in man 

 this axis of the mouth is at right angles to the body axis; but this 

 is not a profound difference, and intermediate conditions are found 

 in some of the higher animals. The most important dissimilarity 

 that needs explanation is, therefore, the existence of the nasal 



Fig. 23. — Median section of the frog. 



a, auricle; an, anus; bl, urinary bladder; 6r, brain; d, cloaca; E.t., Eustachian tube; 

 g.b., gall bladder; int, intestine; kd, kidney; /, liver; la, larynx; Ig, lung; mes, mesen- 

 tery; no, nostril; ovd, oviduct; p, pancreas; pc, pericardium; sp, spleen; sp.c, spinal 

 cord; t, testis; ur, ureter; v, ventricle. 



cavity in vertebrates such as man. As is the case with many 

 other seeming differences of anatomy between the members of the 

 Phylum Vertebrata, this is understandable by reference to their 

 development. In the early stages of a human embryo, as with the 

 embryos of reptiles, birds, and other mammals, there is a single 

 mouth cavity with which the nostrils are connected as in the adult 

 frog. In subsequent development there occur ingrowths from 

 the right and left sides of the upper jaw. These finally meet in 

 the mid-line and divide the mouth cavity into two portions, one 

 of which becomes the nasal cavity. The latter is thus an upper por- 

 tion of the general mouth cavity of the embryo. The adult frog 



