In proceeding to describe these organs, I shall endeavour to shew the manner in which they contribute to perform the function of respiration in each of the four orders of vertebrated animals, beginning with fishes, in which they appear to be simplest ; and I shail confine myself principally to the relation of those facts which have been most lately ascertained, and which appear to establish most clearly the analogy existing between the organs under consideration.

It is from the study of the structure of the respiratory organs, and of the arteries distributed upon them, that the chief part of our knowledge regarding the mode in which the function of respiration is performed by them has been obtained, as little or no direct or strictly 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 fatal life, their variation in size, and alteration in form at different periods, and the changes in the distribution of the vessels supplying them with blood, we are enabled to perceive certain ends to which these changes of structure 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 rudimentary 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 Endochorion, from its lining the chorion. This part is always formed by the expansion of the cloacal part of the intestine, 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.

5. The Amnios, a covering proper to the fœtus, connected with its integuments, and formed by a reflection of the serous layers of the germinal

membrane.

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

In proceeding to describe these organs, I shall endeavour to shew the manner in which they contribute to perform the function of respiration in each of the four orders of vertebrated animals, 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 appear to establish most clearly the analogy existing between the organs under consideration.

It is from the study of the structure of the respiratory organs, and of the arteries distributed upon them, that the chief part of our knowledge regarding the mode in which the function of respiration is performed by them has been obtained, as little or no direct or strictly 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 alteration in form at different periods, and the changes in the distribution of the vessels supplying them with blood, we are enabled to perceive certain ends to which these changes of structure 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 rudimentary 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 Endochorion, from its lining the chorion. This part is always formed by the expansion of the cloacal part of the intestine, 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.

5. The Amnios, a covering proper to the fœtus, connected with its integuments, and formed by a reflection of the serous layers of the germinal

membrane.

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