CHANGES m THE BLOOD. 97 



from tliis cause. This form of congestion becomes aggravated by 

 the blood remaining unchanged, for it is found that when the 

 blood is not properly changed in the lungs, its passage through 

 them is partially impeded, that it accumulates in them, in the 

 right side of the heart, and in the veins generally. 



2. Gravitation. — The veins of the extremities and other de- 

 pending parts of the body are congested from this cause. It 

 indicates an atonic condition of the system generally, with 

 diminished cardiac force ; and is commonly associated with all 

 the exhausting diseases of the horse, more especially if the 

 animal is not exercised. Congestion from gravitation is also 

 caused in many healthy horses by want of exercise. Indeed, a 

 due amount of exercise, when in a condition to take it, seems to 

 be of greater importance to the horse than to horned cattle. 

 The cow or ox wiU remain in the stall for weeks, or even 

 months, tied in one position and receiving no exercise, but tlie 

 limbs never swell, and there are seldom any signs of venous con- 

 gestions ; not so with the great majority of horses. ISTow and 

 then, however, a horse is met with whose limbs never swell from 

 want of exercise, but as a rule horses when not exercised suffer 

 from swellings of those depending parts of the body farthest 

 removed from the centre of the circulation. Other horses 

 habitually suffer from swelled legs, no matter how carefully and 

 properly they are fed and worked. The debility produced by 

 the process of " casting the coat " causes congestion of the vessels 

 of the depending parts. 



A very remarkable example of the effects of gravitation in 

 producing congestion used to be frequently witnessed in the 

 days when bleeding was so commonly performed, and when 

 inflammation of the jugular vein was more frequently met with 

 than now. 



The loss of a jugular from the formation of a thrombus within 

 it, and consequent obliteration of its canal, did not seem to 

 interfere with the cervical circulation until the animal was 

 turned out to gi-ass ; but immediately this was done, the animal's 

 head would commence to sweU to a most alarming extent, 

 proving that, although the collateral circulation was sufficient, 

 so long as the horse was fed from the rack or manger, it was 

 quite insufficient when the head was depressed whilst grazing. 



3. Altered conditions of the blood. — Many congestive diseases, 



H 



