84 A MANUAL OF VETERINARY PHYSIOLOGY 



The capillaries take no part; in fact, as far as is at present known, 

 there is no vasomotor supply to capillaries. The vaso-dilators 

 act by relaxing the muscular coat of the arterioles. 



From what has been previously learnt it is evident that a 

 high blood-pressure is an essential condition throughout the life 

 ol the animal ; whether awake or asleep, at work or at rest, a 

 constant watch has to be kept over the vessels of the body, in 

 order to produce not only an effective but a purposeful circula- 

 tion, for it is evident that the blood-supply to an organ in active 

 secretion must be greater than when the organ is at rest. The 

 nervous system is charged with the duty of controlling the blood- 

 vessels, and has to regulate these matters automatically. 



The system of nerves governing the bloodvessels is not under 

 the control of the will, but belongs to that portion of the nervous 

 apparatus generally known as the sympathetic. The sympa- 

 thetic system has its origin in the brain and spinal cord ; it is 

 specially charged with functions relating to plain muscular 

 tissue, cardiac muscle, and glands, so that the plain muscle 

 constituting one of the coats of the arterioles comes under its 

 control. The two opposite changes in the bloodvessels — viz., 

 contraction and dilatation — are not brought about by the same 

 nerves. The constrictor nerves operate through the elaborate 

 and complex system to which they belong — viz., the sympathetic ; 

 but the dilator fibres accompany cranial and spinal nerves, and 

 may or may not have any connection with the sympathetic 

 system. 



The Constrictor Fibres originate in the medulla at a special 

 part known as the vasomotor centre, and the fibres from the 

 centre pass down the spinal cord in that portion known as the 

 inferior horn of the grey matter. At every segment of the 

 spinal cord between the first dorsal and third or fourth lumbar 

 vertebrae, these nerves flow out of the spinal canal in company 

 with the inferior roots of the spinal nerves, and, leaving the 

 latter, join the ganglia on the sympathetic, which lie just under 

 the arch of the ribs. 



The fibre which joins these constrictors to the sympathetic 

 is the white ramus commnnicans, and it is spoken of as the pre- 

 ganglionic fibre. In the ganglion trie fibre terminates, and a 

 fresh cell in the neighbourhood of its termination gives rise to a 

 new fibre, which leaves the ganglion and is spoken of as the 

 post- ganglionic fibre or grey ramus communicans. This fibre is 

 structurally different from the pre-ganglionic, for it is non- 

 medullated, it passes back to the spinal nerves, and by entering 

 the axillary and sacral plexuses reaches the bloodvessels of the 

 limbs ; those for the skin of the trunk pass by the corresponding 

 spinal nerves. 



