KESPIEATORY SYSTEM. 323 



The two layers now separate ; the superior passes forwards along 

 the roof of the abdomen, and again returns to the posterior part 

 of the diaphragm. The inferior layer descends to and envelops 

 the small intestines, and returns to the abdominal roof, the 

 double layer constituting the mesentery. It now passes back- 

 wards to the iliac region, where it again descends to suspend the 

 floating colon and the rectum, forming the colic mesentery and 

 meso-rectum by returning upwards. In the pelvis it forms the 

 recto- vesical ligament, between the bladder and rectum, finally 

 returning along the inferior parietes of the abdomen to the 

 diaphragm. In the female, the peritoneum, after leaving the 

 rectum, passes to the anterior surface of the vagina and uterus, 

 forming the broad ligaments. 



On the abdominal floor, stretching forwards in the median line 

 from the umbilicus to the quadrate lobe of the liver, is the broad 

 ligament of the latter, and in a similar manner, passing backwards 

 to the fundus of the bladder, we find alsO a broad ligament. 

 Neither of the latter structures are well marked in the adult, 

 especially the last. 



Respiratory System. 



By the action of these organs certain chemical and physical 

 changes take place in the blood, the chief of these consisting in 

 absorption of oxygen from, and giving off carbonic acid to, the 

 atmospheric air, the former change being necessary for the 

 elaboration of the fluid, the latter for the elimination of a sub- 

 stance which, if retained, would prove prejudicial. The organs 

 of respiration are invariably adapted to the wants of the animal 

 and the medium in which it lives. Thus, insects breathe by air- 

 tubes, opening on the surface of the body; in the oyster breath- 

 ing is performed by fringes ; in fishes by gills ; in the mammalia 

 by means of elastic air-receptacles, called lungs, which are 

 enclosed in special cavities, and communicate with the atmos- 

 phere by means of an air- tube. 



In the horse, who breathes only through his nose, the organs 

 of respiration are the Nostrils, Nasal Chambers, Pharynx, Larynx, 

 Trachea, and in the thoracic cavity the Bronchi, Bronchial tubes, 

 and Lungs. 



THE nostrils. 



The nasal openings, or nostrils, are two, right and left, oblong 

 -openings, situated in the anterior part of the face at the 



