570 ORGANS OF RESPIRATION. 



pieces of the thyroid cartilage of birds, have united to the 

 cricoid,) dilate the entrance of the larynx. Now that the 

 epiglottis is a distinct, detached and moveable part, the ary- 

 epiglotticus, which in inferior classes acted only on the 

 arytaenoid cartilages, serves in mammalia, to depress the 

 moveable epiglottis. The crico-arytcenoideus lateralis has 

 also shifted from the thyroid to the cricoid cartilage, and 

 serves to compress the passage of the larynx, and a similar 

 function is performed by the thyrio-arytcenoideus, and the 

 arytaenoideus transversus and obliquus. And with this elastic, 

 vibratile, and muscular apparatus, developed at the entrance 

 of the respiratory organs of air-breathing vertebrata, they 

 are enabled to regulate the quantity of air sent to or from 

 the lungs, to protect these organs against the intrusion of 

 foreign matter from the buccal or nasal cavities, and to 

 vocalize the air transmitted through the trachea, so as to 

 produce the various sounds of animals. 



The development and metamorphosis of the branchial 

 arteries, and the successive formation and closing of the 

 branchial openings of the neck in the mammalia, present the 

 same phenomena as in the inferior air-breathing vertebra ted 

 classes, and have often been observed and described in the 

 human body, as well as in inferior quadrupeds, by Rathke, 

 Baer, Ascherson, Burdach, Meckel, Miiller, and other 

 anatomists. The branchial arterial arches are successively 

 formed from before backwards, and successively metamor- 

 phosed into the ascending cephalic and brachial arterial 

 trunks, the descending aorta, and the pulmonary arteries, 

 but no true branchiae have yet been detected in this class, 

 nor above the amphibia. Three or four branchial openings 

 on each side of the neck, are commonly observed present at 

 the same time, in mammalia, as in the lower classes ; they 

 appear and disappear in succession from before backwards, 

 and the anterior openings on each side are generally much 

 larger than the two posterior. In a human embryo three 

 lines long, Rathke found the sides of the neck not yet per- 

 forated by the branchial openings ; in the hog, they were 

 open in an embryo only six lines long, also in an embryo of 

 the horse only eight lines in length, and at an early period 

 in the embryo of the sheep. Baer perceived three pairs of 

 branchial openings, the anterior pair being much larger than 

 the posterior, in a human embryo at the fifth week; and in 



