6o HUMAN ANATOMY. 



apparatus of water-breathing vertebrates, in which the respiratory function is per- 

 formed by means of the rich vascular fringes hning the clefts through which 

 the water passes, thus permitting the exchange between the oxygen of the water 

 and the carbon dioxide of the blood. Each arch is supplied by a blood-vessel, or 

 aortic bow, which passes from the main ventral stem, the truncus arteriosus, through 

 the substance of the visceral arch backward to. unite with the similar bows to form 

 the dorsal aortce. In aquatic vertebrates the aortic bows supply an elaborate system 

 of secondary branchial twigs, which form rich capillary plexuses within the gills ; in 

 air-breathing vertebrates, however, in which these structures are only rudimen- 

 tary, the main stems, the aortic bows, are alone represented. With the loss of func- 

 tion which follows the'acquisition of aerial respiration in the higher vertebrates, the 

 number of visceral arches is reduced from six, or even seven, as seen in fishes, to 

 five, the fifth arch in man, however, being so blended with the surrounding struc- 

 tures that it is not visible externally as a distinct bar. In their condition of great- 

 est perfection, as in fishes, each visceral arch contributes an osseous bar, which 

 forms part of the branchial skeleton ; these bony bars are represented in man and 

 mammals by cartilaginous rods, which temporarily occupy the upper arches, for the 

 most part entirely disappearing. When viewed in frontal section (Fig. 73), the 

 mammalian visceral arches are seen as mesodermic cylinders imperfectly separated 

 by external and internal grooves, the visceral furrows and the pharyngeal pouches 

 respectively ; this arrangement emphasizes another modification following loss of 

 function, — namely, the conversion of the true visceral clefts of the lower forms into 



furrows, — since in man and mammals the 

 Fig. 72. fissures are closed by the occluding mem- 



brane formed by the apposition of the 

 ectoblast and the entoblast at the bottom 

 of the outer and inner furrows. 

 ^, ... The First or Mandibular Arch 



Maxillarx ,> ,. ...... . 



process — ^r;r:---4a ' f o j i. early becomes differentiated into a short 



p . • • ^^^Rjj, /_x_ J. .Second arch -^ . , 



orai'cavuy "^ // Third arch "PP^^ ^r maxillary proccss and a longer 



Mandibular ..,^ • J ., XowQv OX mandibular proccss. The maxil- 



process ~ -^  -r— Fourth arch , • •  ••, •. r 11 



^ y -\ . lary process, in conjunction With Its fellow 



„,^_j^ 1 arc ^j ^j^^ opposite sidc and the fronto-nasal 



Head of human embryo of about twenty-one days hyoceSS, which descends as a median prO- 

 seen from the side, showmg visceral arches and external { . \ „. ^ .. 



visceral furrows. X 20. {After His.) jection from the head ( Fig. 75), Contrib- 



utes the tissue from which the superior 

 and lateral boundaries of the oral cavity and the nasal region are derived. The 

 mandibular process joins with its mate in the mid-line and gives rise to the lower 

 jaw and other tissues forming the inferior boundary of the primary oral cavity. The 

 latter in its original condition appears as a widely open space leading into the primi- 

 tive pharyngeal cavity ; later the septum is formed which divides the oral from the 

 nasal cavity. The mandibular process contains a cartilaginous rod, which for a 

 time represents the corresponding bony arch of the visceral skeleton of lower types. 

 The ventral and larger part of this rod, known as Meckel s cartilage, entirely disap- 

 pears, the lower jaw being developed independently around this bar of cartilage ; 

 the upper end of the cartilaginous bar, however, persists and forms two of the ear- 

 ossicles, the malleus and the incus. 



The Second or Hyoid Arch also contains a cartilaginous bar, from the ven- 

 tral segment of which (known as the cartilage of Reichert) is derived the smaller 

 cornu of the hyoid bone ; the dorsal end of the bar, which is fused with the tem- 

 poral bone, gives rise to the styloid process, the intervening portion of the cartilage 

 persisting as the stylo-hyoid ligament. The cartilage of the second arch is also con- 

 cerned in the formation of the stapes. The origin of this ear-ossicle is double, since 

 the crura of the stapes are derived from the cartilage of the hyoid arch, while the 

 base is contributed by the general cartilaginous capsule of the labyrinth. The char- 

 acteristic form of the stapes is secondary and due to the perforation of the triangu- 

 lar plate, the early representative of this bone, which thus acquires its characteristic 

 stirrup-shape in consequence of the penetration of a minute blood-vessel, l\\^ perfo- 

 rating stapedial artery, a branch of the internal carotid, which later disappears. 



