284 A Manual of Veterinary Physiology. 



Colin draws attention to the difficulty in producing 

 paralysis experimentally in the horse from lesions of the 

 hemispheres. Neither the artificial production of a clot in 

 the falciform sinus, nor the introduction of pieces of lead 

 the size of a pea into the convolutions, gave rise to 

 hemiplegia. This quite bears out what we know to be a 

 clinical fact, that horses may have in their lateral ventricles 

 tumours the size of an egg without producing any disturb- 

 ance. I have seen many such cases, the tumours being of 

 variable size, and the clinical history has never given more 

 than a few days' illness, though the growths must have 

 been forming for a considerable period. 



The circulation in the brain is peculiar. The veins, or 

 so-called sinuses, are enclosed in very rigid membranous 

 walls, the blood being driven through them, not only by 

 the force behind, but by the aspiratory effect produced by 

 inspiration (see p. 74). 



The fluid found in the ventricles is a secretion rather 

 than a transudation. The cavity of the ventricles com- 

 municates, by means of a foramen in the roof of the 

 fourth ventricle, with the central canal of the spinal 

 cord. The amount of fluid in the ventricles is normally 

 80 grains to 90 grains, but in disease may be as much as 

 7 ozs. to 10 ozs., according to Colin, who examined the 

 brain of several horses suffering from immobilite, and says 

 that in each case he found an excess of fluid in the 

 ventricles. 



For the peculiarities in the cerebral circulation, and the 

 use of the cerebro-spinal fluid, see p. 73. 



Nothing is known of the lymphatics of the brain. 



Cranial Nerves. 



These are divided into the nerves of special sense, 

 sensory nerves, motor nerves, mixed nerves. Altogether they 

 make twelve pairs, and all but Nos. 1, '2 and :> arise from 

 the medulla. 



For nerves Nos. I and 2 see the Senses. 



Third Pair, or Motor Oculi, is one of the motor nerves of 



