PATHOLOGY OF PULMONARY DISEASES OF THE HORSE. 
19 
when the union with the mother by means of the funiculus um- 
bilicalis still continues, the supply of oxygen from the mother 
ceases, on account of the contraction of the uterus; the contrac¬ 
tion of the uterus conditions a compression of the vessels. A 
proof of the correctness of this assertion, is given by the obser¬ 
vation that the foetus begins to respire, and aspires the liquor 
amnii, if the fun. umbilicalis be compressed. The supply of air 
to tlie lungs is only possible, however, when the passage to the 
same is open. When the last is completely closed, no respiration 
whatever can take place, and the young dies during or at once 
after birth. If parts of the respiratory-tube are obstructed, then 
the young cannot breath with the parts of the lungs in connection 
with same, such parts remain empty atelectatic. This is <%w-geni- 
tal atelectasis, that is, the lungs or single sections of the same 
remain in the foetal condition. 
The mechanical acts of respiration are also important in consid¬ 
ering the process of respiration. These are the respiratory move¬ 
ments. Different powers serve to perform the business of in- and 
expiration, to which it is necessary for us to give a moment’s 
attention. . The lungs lie in relation with the inner surface of the 
thoracic parietes. When the thorax widens, the lungs distend 
themselves, and the distention of the lungs exactly corresponds to 
the extension of the thorax. A dilution of the air in the lungs 
must then take place in consequence of the extension of the 
thorax, and the succeeding distension of the lungs, upon which 
follows an in-streaming of the external air by means of the res¬ 
piratory-tube. This in-streaming of the external air in the lungs 
continues until the equilibrium, or, as Donders remarks, until an 
equal tension of all the alveolae, and an equal distribution of the 
pressure to all parts has taken place. The lungs do not, there¬ 
fore, deport themselves by inspiration in an active, but in a pas¬ 
sive manner; they are extended by means of the atmospheric 
pressure, and resist the same by means of their elasticity. This 
resistance, which the pulmonic tissues offer to the atmospheric 
pressure; that is, the endeavor of the lungs to contract or retain 
themselves in their natural volume, is called the negative pressure. 
The inspiration is interposed by means of muscular force, the 
