I02 THB HORSB. 



Structure. It is composed chiefly of two structures; the gray, 

 originating impulses and receiving impressions, and the white, conduct- 

 ing impressions. 



Cerebro-spinal System. This consists of the brain, spinal 



GANGLIA, CRANIAIv NERVES, and SPINAL NERVES. 



Membrane Surrounding- the Brain. The membrane which 

 Unes the skull constituting an interior periosteum is called the dura 

 mater; beneath this is a delicate membrane called the arachnoid, and 

 still beneath this is another membrane, which even dips into the con- 

 volutions of the underlying brain. This is called the pia mater. These 

 three divisions taken together are called the meninges. 



Divisions of the Brain. The divisions of the brain are the 



CEREBRUM, CEREBELLUM, PONS VAROLII and MEDULLA OBLONGATA. 



Functions of the Cerebrum. "The cerebral hemispheres are 

 the organs by which perception is carried on and from which motor im- 

 pulses are given out. They contain the organ of the will; they possess 

 memory, or the means of retaining impressions of sensible influences. ' ' 

 (Hare.) 



Functions of the Cerebellum. "It is absolutely insensible to 

 irritation and may be cut away without any signs of pain; its removal 

 from the body or destruction by disease is generally unaccompanied by 

 loss or disorder of sensibility. Animals from which it is removed can 

 see, hear, and feel pain to all appearance as perfectly as before. It 

 governs the coordination of movements, and while irritation of the cere- 

 bellum produces no movements at all, remarkable results are produced 

 by removing part of its substance. As portion after portion of it is cut 

 away the animal gradually loses the power of springing, walking, stand- 

 ing, or preserving its equilibrium. If laid upon its back it cannot re- 

 cover its normal posture but struggles to get up, and if a blow is threat- 

 ened tries to avoid it, but fails to do so. According to Gowers, the 

 middle lobe of the cerebellum governs equilibrium by means of afferent 

 fibres from the semi-circular canals and the ocular muscles and also the 

 muscles of the legs." (Hare). 



Function of the Pons Varolii. "It contains a large number of 

 nerve fibres both transverse and longitudinal, and is a conductor of im- 

 pressions from one part of the spinal axis to another. Concerning its 

 functions as a nerve centre little or nothing is certainly known." 

 (Hare). 



