470 A MANUAL OF VETERINARY PHYSIOLOGY 



peduncle. The fibres terminate by arborising around cells in 

 the middle lobe of the organ. 



Association Fibres. — Certain short fibres, previously mentioned, 

 arise in the cord, and form tracts uniting the various segments. 

 They originate from ' tract cells ' in the grey matter of the 

 cornua, and from here run into the white substance, and after a 

 short course they run back and re-enter the grey matter. These 

 tracts may be ascending or descending ; their function is to 

 knit together the various spinal segments. 



The above ascending and descending tracts do not account 

 for the total amount of white matter ; there are still unmapped 

 regions in the cord. 



When the various ascending tracts reach the medalla, they 

 undergo change in form, position, and distribution. Some, as 

 we have seen, terminate in cells in the bulb — in fact, there are 

 only two upward paths which pass through unbroken — viz., the 

 two cerebellar. In the descending tracts, which are passing from 

 the cerebrum and cerebellum to the cord, only one — the pyra- 

 midal tract — passes through unbroken. All the others behave 

 as those of the ascending tracts — viz., terminate in a collection 

 of cells — spoken of as ' nuclei ' — and from these fresh fibres 

 arise. 



Functions of the Ascending Tracts. — In the median superior 

 and lateral superior tracts and the dorsal columns of the cord im- 

 pulses are conveyed which lead to a sensation of pressure (touch) , 

 but more especially impressions from skeletal muscles, tendons, 

 and joints, known as muscle-sense, to which is due the co-ordina- 

 tion of muscles in locomotion. It will be remembered that the 

 lateral superior tract is almost wholly composed of sensory fibres 

 from the muscles. These impulses are carried up the same side 

 of the cord on which they entered. In the bulb the paths end, 

 fresh fibres are formed, and cross to the opposite side, to terminate 

 in the cerebrum, in the area know T n as that of the body-senses. 

 Injury to the fibres of body-sense produces an awkwardness in 

 gait ; the animals show a want of skill in using the hind-limbs. 

 The muscles are there, but, owing to the loss of muscle-sense, 

 they do not know it, and consequently are unable to direct their 

 movements. 



The dorsal cerebellar tract and the ventral cerebellar tract carry 

 impulses connected with deep sensibility, arising from muscles, 

 tendons, and joints. Each of these tracts terminates in the cere- 

 bellum ; it is believed they are also associated with muscular 

 co-ordination, which, as we shall see later, is the special function 

 of the cerebellum. 



The lateral superior tract is the path for the transmission of 

 impulses giving rise to sensations of pain, heat, and cold ; 



