340 DISEASES AFFECTING THE CEREBRAL CIRCULATIOX. 



Another set of cases very apt to be confounded with these, 

 from the similarity of the symptoms exhibited, are those of 

 cerebral disturbance, met with in all classes of horses, irrespec- 

 tive of the nature of their employment, whether in or out of 

 draught, but owing their origin to very different causes, viz. 

 organic changes in connection with certain cerebral structures. 



b. Causation. — Hyperemia of the brain-structure may occur 

 from more than one cause. It may be associated with general 

 plethora, with increased cardiac power or action, with disturbed 

 vaso-motor activity, or with any interference with the general 

 arterial or capillary circulation, as a result of which more blood 

 is sent to the cranial structures — active hypercmiia. Or it 

 may follow as the result of interference with the natural 

 escape of blood from the cranial vessels — mechanical hyper- 

 cemia. 



There seems little doubt that the latter is the main factor in 

 operation here, that all those cases of megrims or cerebral con- 

 gestion occurring in harness-horses — excepting, of course, such 

 as may be associated with organic lesions of the brain — are 

 most rationally accounted for by attributing them to obstructed 

 venous circulation, occasioned by the pressure of tight or badly 

 fitting neck-collars. No doubt the pushing of an animal 

 beyond his natural pace in going up a hill immediately after a 

 full meal, or exposure to the rays of a mid-day sun, may all 

 have a tendenc}^ to hasten the appearance of the symptoms, 

 or aggravate them when occurring ; but none of them, indi- 

 vidually or taken together, independent of the collar-pressure 

 is sufficient to produce this vertigo. This view, however, is not 

 acquiesced in by all, many regarding it as inflammatory in its 

 nature, and the result of the operation of other general or local 

 inflammatory inducing agencies. This we cannot understand, 

 seeing that neither in its mode of origin, termination, nor any 

 part of its course, is there resemblance to inflammatory action. 

 Again, it has been said, were this collar-pressure the true cause 

 of its origin there would be many more cases in harness-horses 

 than actually do occur. This does not appear so clear and in- 

 controvertible as some would have us to believe ; for it ought to 

 be borne in mind that it is, even amongst harness-horses, a 

 small minority Avhich have so peculiarly formed necks, or carry 

 their heads in such a style that, with even an indiflbrently fitting 



