loo OBSTETRICAL PHYSIOLOGY. 



the organs of respiration, and rapidly increase in capacity, while the 

 thorax enlarges in a commensurate degree ; the pulmonary artery also 

 dilates to admit the increased flow of blood, and the ductus arteriosus is 

 obliterated to prevent the mixture of arterial and venous blood. The 

 ductus venosus also aids in the isolation of the two kinds of blood by 

 becoming atrophied, as does also the occlusion of the foramen ovale. 

 This opening, however, not infrequently remains intact in young animals ; 

 though, as a rule, this does not greatly affect the circulation, for when 

 the heart contracts the auricles are isolated by the narrowing of the 

 orifice and the elevation of a valve. 



The Respiratory Apparatus. 



There is an absence of unanimity as to the mode of development of the 

 respiratory apparatus, and particularly the lufigs. Some assert that the 

 latter are derived from two little distended cellular masses attached to 

 the anterior part of the intestinal tube, and which afterwards become 

 perforated with numerous ramifying cavities, that communicate with the 

 trachea ; while others describe them as commencing by a median enlarge- 

 ment, which is hollow, and opens into the cesophagus. The walls of the 

 orifice of communication with the digestive passage become considerably 

 lengthened, and afterwards form the trachea and larynx ; while the 

 vesicle or enlargement representing the lungs divides into two pyriform 

 sacs, each of which is greatly subdivided to constitute the pulmonary 

 lobes, with their vesicles and infundibula. The trachea is completed 

 through the formation of the cartilaginous rings in the tube that attaches 

 the lungs to the oesophagus ; the larynx is developed in the same man- 

 ner, at the pharyngeal opening of the tube. This organ, however, is 

 not very distinguishable during foetal life, and only assumes its definitive 

 form and volume at puberty. 



Up to birth the placenta retains the function of the lungs, which, though 

 ready to act, only come into play when the creature is born. Previous 

 to this event they are of a dark red color, firm and compact, heavier 

 than water, and apparently destitute of alveoli ; though these latter exist, 

 but are filled with embryonic elements, and their walls are in contact. 

 A moderate insufflation is sufficient to distend the air-vesicles, when the 

 lungs become crepitant and enlarged, have a rosy color and spongy 

 appearance,, float in water, and the air cannot be completely expelled 

 from them. The same change immediately occurs in these organs when 

 the young creature is born alive ; the external atmosphere, acting upon 

 the surface of its body, causes it to inspire deeply, the chest dilates, the 

 air rushes into the lungs, and respiration commences, only to cease with 

 life. This alteration in the color, texture, and specific gravity of the 

 lungs enables, us to decide, in certain cases, whether or not an animal 

 has been born alive. 



The thymus gland first appears towards the second month, as a growth 

 from the respiratory mucous membrane, near the larynx ; it then descends 

 gradually along the trachea until it reaches the thorax, where it is situated 

 between the layers of the anterior medianastinum. It increases in size 

 until birth, after which it remains stationary for a short time ; then it 

 gradually diminishes and disappears at a period which varies according 

 to species, and even individuals. Exceptionally, it has been found in 

 horses three years of age. It is a gland in structure, though it has no 

 excretory canal. Its uses are unknown, though it is surmised that it plays 



