426 A MANUAL OF VETERINARY PHYSIOLOGY 



which no better example could be given than that of the vagus, 

 with its fibres to the larynx, lungs, heart, stomach, and intes- 

 tines, each functioning in its own way and independently of its 

 neighbour, with which it may have no more in common than 

 if it never existed. 



Nerves are remarkable for their want of elasticity, but they 

 are capable of very considerable stretching without breaking. 

 In man the nerves of the limbs require a weight of from i8-2 kilo- 

 grammes (40 pounds) to 54-6 kilogrammes (120 pounds) to break 

 them. Nerves are also very indifferently supplied with blood- 

 vessels. 



Nerve Terminations. — There are some structures, such as 

 glands, where the nature of the nerve termination is not satis- 

 factorily made out ; there are other places, such as muscle, 

 where definite and distinct motor nerve-endings have been 

 found ; and on many sensory and sympathetic nerves special 

 terminations, known as ' Pacinian corpuscles ' and ' Krause's 

 end-bulbs/ exist. Nerve terminations are found in the muzzle 

 of animals, in tendons, in muscles, in the generative organs, 

 conjunctiva, mouth, tongue, epiglottis, etc. ; some are known as 

 1 Krause's end-bulbs,' those in tendon are described as the ' organ 

 of Golgi,' in muscle they are known as ' end-plates,' whilst in the 

 skin of the muzzle the nerves terminate in small swellings or 

 enlargements known as ' tactile cells,' which are placed between 

 the epithelial cells of the epidermis ; cells of this kind also exist 

 in the foot of the horse. The nerves of special sense have each a 

 distinct termination peculiar to themselves, such as the hair-cells 

 of the internal ear, the rods and cones of the retina, taste-bulbs 

 of the tongue, etc. 



Chemistry of Nerve-Fibres. — The chemistry of these tissues is 

 very imperfectly known. Advances in physiological chemistry 

 have shown that substances like protagon, which at one time was 

 believed to represent the essential composition of the medullary 

 substance or myelin, are really a mixture of substances. The 

 myelin furnishes three substances which possess definite chemical 

 characteristics — viz., cholesterin, lecithin, and cerebrosides. 

 Cholesterin is a substance containing neither nitrogen nor phos- 

 phorus, and in chemical nature is allied to a group of bodies 

 found in plants known as terpenes. Its silvery crystalline for- 

 mation is characteristically shown in the tumours on the choroid 

 plexus of the horse. In the body it occurs with lecithin, though 

 the nature of the physiological connection, if any, is unknown. 

 Lecithin is a phosphorus-containing nitrogenous fat, especially 

 characteristic of the nervous system, but found elsewhere in the 

 body. When lecithin is decomposed it yields, among other 

 products, a fatty acid ; it is this which is blackened in osmic acid 



