92 Dr Allen Thomson on the Vascular Si/stem 



. In proceeding to describe these organs, I shall endeavour to 

 shew the manner in which they contribute to perform the func- 

 tion of respiration in each of the four orders of vertebrated ani- 

 mals, beginning with fishes, in which they appear to be simplest ; 

 and I shall .confine myself principally to the relation of those 

 facts which have been most lately ascertained, and which ap- 

 pear to establish most clearly the analogy existing between the 

 organs under consideration. 



It is from the study of the structure of the respiratory or- 

 gans, and of the arteries distributed upon them, that the chief 

 part of our knowledge regarding the mode in which the func- 

 tion of respiration is performed by them has been obtained, as 

 little or no direct or ^itrictly physiological evidence has hitherto 

 been procured from the observation of their mode of action. 

 The description of these organs indeed might be considered as a 

 subject merely anatomical, were it not that in observing their 

 development during foetal life, their variation in size, and alte- 

 ration in form at different periods, and the changes in the dis- 

 tribution of the vessels supplying them with blood, we are en- 

 abled to perceive certain ends to which these changes of struc- 

 ture are directed, and thus acquire some insight into the mode 

 of operation of each of them. 



It is now well known that the constant presence of oxygen in 

 some form or other is absolutely necessary for the development 

 of the embryo of all animals. The evolution of those rudi- 

 mentary parts even, the formation of which precedes that of the 



as well as in the tortoise. It is called Chorion by Emmert in the lizard, 

 and by Pander in the bird ; Umbilical vesicle by Haller in the bird, in 

 which he was the first to shew its connection with the funis of the urachus. 

 In Mammalia it has been called Endoohorion, from its lining the chorion. 

 This part is always formed by the expansion of the cloacal part of the intes- 

 tine, and carries upon it the ramifications of the umbilical arteries and veins. 



3. The internal gills are those formed in the course of the branchial plates 

 or hoops which surround the pharynx. 



4. The external gills are appendages of the foregoing, connected generally 

 with the outer part of the branchial plates, and receiving a covering from the 

 integuments. 



6. The Amnios, a covering proper to the foetus, connected with its inte- 

 guments, and formed by a reflection of the serous layers of the germinal 

 membrane. , 



6. The Chorion, the external envelope of the whole ovum. 



