ORGANS OF RESPIRATION. 



a dog's embryo of three weeks, he detected four branchial 

 arteries co-existing on each side, with the minute rudiment 

 of a posterior fifth pair. Four pairs of these openings were 

 observed present at the same time, by Miiller and by Bur- 

 dach, in the human embryo about the fourth week ; Rathke 

 found three pairs of unequal openings in the human embryo, 

 about the sixth week; and Ascherson has observed arid 

 described several abnormal cases, where remnants of these 

 branchial openings were preserved through life, forming 

 permanent congenital fistulas in the human neck. 



The same simple and uniform plan, followed by nature in 

 the construction of all animal organs, is not less apparent 

 in the development of the pulmonary organs of man and 

 mammalia, than in that of their branchial apparatus. The 

 lungs begin by a single simple follicle, as other glands, 

 originating on the median plain, in a vascular blastema, 

 on the fore part of the oesophagus, and soon extend down- 

 wards, bifurcate, and enlarge at their closed extremities, so 

 as to constitute two simple pulmonary sacs, with a single 

 membranous trachea, like that of fishes, or the lowest am- 

 phibia, or a lepidosiren. Peripheral septa begin to appear 

 on the inner surface of their dilated part, the rudiments of 

 future terminal cells. The trachea elongates, and irregular 

 continuous spots of cartilage are perceived extending on its 

 two sides, which rudiments of tracheal rings as yet neither 

 meet in front nor behind, a condition preserved in the adult 

 pulmonary organs of many amphibia, and the lowest reptiles. 

 The peripheral septa extend inwards, to cancellate the pul- 

 monary sacs, proceeding from their proximal towards their 

 distal ends, as in reptiles. The lateral tracheal primitive 

 cartilages, now extending their development upwards and 

 downwards along the sides of that tube, become divided 

 transversely, by absorption, into separate pieces, to form 

 imperfect and incomplete tracheal rings. By the gradual 

 approximation of these imperfect rings, on the median plain, 

 before and behind, by their increased size and number, and 

 by their enlargement and coalescence at the beginning of 

 the air- tube, the annulated trachea and the laryngeal carti- 

 lages are distinguished and completed, as the uncancellated 

 membranous tubular bronchi had been previously rendered 

 distinct from the capaceous cancellated lungs. The tracheal 



