THE HEAD AND NECK tvjx 



lateral lobe of the thyroid body is developed. It also gives rise to the sinus 

 Pyriformis of the larynx. 



Visceral or Branchial Arches. — Between the pharyngeal ponches a thicken- 

 ing of mesoderm takes place in the intervals between the entoderm and 

 ectoderm. These thickenings give rise to elevated, rounded bars, which 

 constitute the visceral or branchial arches. The number of these arches on 

 either side is six. The sixth, however, which lies behind the fifth pharyngeal 

 pouch, remains undifferentiated. Like the pharyngeal pouches, the visceral 

 arches are arranged in pairs. The dorsal extremities of the arches, right 

 and left, stand apart, and are attached to the sides of the head. The ventral 

 extremities, on either side, approach the mesial Une of the future embryonic 

 neck, and subsequently they become connected along the mesijd hne. Each 

 arch projects into the pharyngeal part of the fore-gut, and the first four also 

 form projections on the exterior, which increase the depth of the external 



Tuberculiun Impar 

 Furcula 



Fig. 523. — The Visceral Arches of the Embryo (Posterior View) (His). 



I. Mandibular .Arch III. Thyro-hyoid Arch 



II. HyoidArch IV. Fourth Arch 



V. Fifth Arch 



furrows, corresponding to the visceral clefts. The names of the arches, from 

 the cephaUc region caudalwards, are as follows : 



First, or mandibular, arch. 



Second, or hyoid. arch. 



Third ] 



Fourth I „ , . , . 



Fifth I Branchied arches proper. 



Sixth j 



The first arch on either side is situated between the first pharyngeal pouch 

 and the stomodaeum or primitive oral ca\ity. By means of their mandibular 

 and maxillary processes, right and left (the latter being supplemented by the 

 fronto- nasal process), the first arches support the primitive oral cavity, 

 forming its lower or posterior eind upper or iinterior boundaries. They are 

 therefore known as the oral arches, the succeeding arches being post-oral. 



Each visceral arch, in its primitive condition, consists of (i) a central 

 portion, or core, and (2) an epitheUal investment. The core consists of 

 mesenchyme (mesoderm), and is peculiar to its own arch. The epitheUal 

 investment is derived internally from the entoderm of the pharyngeal part 

 of the fore-gut, and externally firom the ectoderm of the exterior. 



At a later stage each visceral arch acquires the following four elements: 

 (i) An artery; (2) a cervical myotome or muscle-segment; (3) a nerve (or 

 nerves) ; £ind (4) a rod of cartilage. 



The artery is known as a visceral-arch artery, and it is one of the primitive 

 aortic arches, which arches, on either side, estabUsh a communication between 

 the corresponding right and left primitive ventral and dorsal aortae. 



