3 so A MANUAL OF VETERINARY PHYSIOLOGY 



being possibly due to the loss of trophic influence, though much 

 may be said in support of the view that the effects observed may 

 be due to failure of the mechanically protective arrangements 

 of the parts affected, the failure resulting from section of the 

 merely motor and sensory fibres which the respective nerves 

 contain. 



But disordered nutrition of a tissue may show itself without 

 any obvious injury to trophic nerves — as, for example, in the 

 phenomenon known as inflammation, or the well-known sym- 

 pathy existing between the digestive system of the horse and 

 the laminae of the feet. Further evidence of nervous action is 

 afforded in nutrition which is norma] in character, such as the 

 change of the coat with the season of the year. The influence 

 of light on metabolism is also probably effected through the 

 nervous system. It is generally considered that a connection 

 between visual sensations and the nutrition of the skin exists in 

 blind men and animals, and the popular belief that a blind horse 

 carries a heavy coat in summer and a short one in winter may 

 be something more than mere superstition. In making these 

 statements we must guard against the error of considering that 

 no growth, repair, or reproduction can take place excepting under 

 the influence of the nervous system. The trophic influence exer- 

 cised by nerves appears to be directed to maintaining in equili- 

 brium the processes of building up and breaking down which are 

 occurring in all tissues. Though the metabolism of the body is 

 largely regulated by the nervous system, yet the process cannot 

 be carried out without food. It is true that metabolism goes on 

 during starvation, but even then food is being supplied, inasmuch 

 as the animal is living on its own tissues. 



The food must contain the elements required by the tissues — 

 viz., water, protein, fat (or carbohydrate), and salts. Each of 

 these must be in proper proportion, neither deficient nor in 

 excess of the animal's requirements ; each must be present, fat 

 cannot be substituted for proteid, nothing can take the place of 

 salts, and a water-free diet sustains life less long than does the 

 entire absence of food as long as water is consumed. We have, 

 therefore, to inquire why it is these substances are absolutely 

 essential in every diet, and how they behave in the system. 



Nitrogenous Equilibrium. — The storing up of protein is an ex- 

 pression which has repeatedly occurred during the previous 

 chapters, and a natural impression may have been created that 

 of the total protein in a diet some of it is daily stored up, so that 

 less leaves the body than entered it by the mouth. There can 

 be no doubt that some of the daily protein of the diet is stored 

 up, but the greater part is not. There can be no more familiar 

 fact than that the body may continue from month to month at 



