108 OBSTETRICAL FHYSIOLOGY. 



perforated with numerous ramifying cavities, that communicate with 

 the trachea ; while others describe tliem as commencing by a median 

 enlargement, which is hollow, and opens into the oesophagus. 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 manner, at the pharyngeal opening of the tube. This organ, 

 however, is not very distinguishable during foetal life, and only assumes 

 its definite 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 colour, 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 insuffiation is sufficient to distend the air- 

 vesicles, when the lungs become crepitant and enlarged, have a rosy 

 colour 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 colour, 

 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 mediastinum. 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 struc- 

 ture, though it has no excretory canal. Its uses are unknown, but it 

 is surmised that it plays a part in the nutrition and haematosis of the 

 foetus and young animal. It may be that, like the spleen, it assists in 

 converting the white corpuscles of the blood into red corpuscles. 



The Digestive Apixiratus. 



The development of the digestive apparatus comprises the formation 

 of the alimentary canal and the organs attached thereto. The alimen- 

 tarij canal begins to appear after the first outlines of the nervous centres 

 and the vascular apparatus have been manifested. We have already 

 described the manner in which the intestinal cavity was formed from 

 the inner lamina of the blastoderm. This cavity, for convenience of 

 description, may be divided into three portions : the anterior intestine, 

 which originates the pharynx and oesophagus ; the middle intestine, 

 which becomes the stomach and intestines proper ; and the posterior 

 intestine, which constitutes the rectum. The chief, or middle intestine, 

 is at first a cylindrical uniform tube, the diameter of which is after- 



