1 62 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



cartilaginous or bony gill arch, already described in connection with the skeleton. Certain blood 

 vessels, the aortic arches, also traverse the visceral arches. The visceral muscles, already 

 described, develop from the tissue of the visceral arches. During the change from the aqua- 

 tic to the terrestrial habit of life the visceral clefts, pouches, arches, and their derivatives, 

 undergo profound alterations. 



e) The tympanic cavity and the external auditory meatus: Beginning with Amphibia the 

 first visceral pouch forms a pouchlike outgrowth in the direction of the internal ear. The end 

 of this outgrowth expands into a chamber, the tympanic cavity or cavity of the middle ear. The 

 stalk of the outgrowth forms the auditory or Eustachian tube, connecting the pharyngeal cavity 

 with the tympanic cavity. In some reptiles, in birds, and mammals an invagination occurs 

 corresponding to the position of the first external gill slit. The bottom of this invagination 

 meets the wall of the tympanic cavity forming at the place of contact a membrane of double 



FIG. 47. Diagrams to illustrate the formation of the visceral pouches, furrows, arches, and clefts. 

 A, early stage, showing the evaginations a from the wall of the pharynx b, which form the visceral 

 pouches. B, later stage, illustrating formation of the visceral furrows c by invagination of the surface 

 ectoderm. C, later stage showing formation of the external gill slits g by the union of the visceral 

 pouches and furrows and breaking through of the points of union, a, visceral pouch; b, pharyngeal 

 cavity; c, visceral furrow; d, tympanic cavity; e, thyroid gland; /, internal gill slit; g, external gill 

 slit; h } visceral arch; i, ectoderm; j, entoderm. 



origin, the tympanic membrane (ear drum). The passage formed by the invagination is the 

 external auditory meatus. 



/) Glandlike derivatives of the visceral pouches: In the adults of land vertebrates the 

 visceral pouches and clefts disappear, leaving, however, certain bodies which are produced 

 by proliferation of the lining epithelium of the visceral pouches. These glandlike bodies, 

 remnants of the visceral pouches, are very variable in number and mode of origin in different 

 vertebrates. Among them may be mentioned: the true or palatine tonsils, from the second 

 visceral pouch, and the thymus, parathyroids, and the postbranchial or epithelial bodies, from 

 a variable number of visceral pouches. The three last named are glandlike bodies which persist 

 in the neck region and appear to belong to the category of the glands of internal secretion, 

 although their function is uncertain. These glands are present although imperfectly developed 

 in fishes. 



g) The trachea and lungs: When the vertebrates adopted the land habitat, the gill slits 

 and gills disappeared in the adult, and their physiological role was taken over by a new out- 

 growth from the pharynx. This outgrowth is a tube which evaginates from the midventral 



