116 THE DIGESTIVE SYSTEM. 



Oil each side of the tube, joined together by a narrow band, or 

 isthmus. These glands are termed the thymus or thyroid bodies. 

 They are large in foetal life, but tend to diminish in size as the 

 animal matures in age; they are well supplied with blood vessels, 

 nerves and lymphatics, and although their use is not fully de- 

 termined upon by anotomists, they doubtless exert an important 

 influence, especially in young animals. It is these glands which 

 become enlarged in goitre, a condition which we shall consider 

 later on in this work. 



Vhe Digestive Organs in General. 



These are more extensive and complicated than the corres- 

 ponding parts of the canivorous animals, as they have a more 

 onerous task to perform, the food being crude and wholly un- 

 prepared as compared to the flesh consumed by the carnivora, 

 which virtually is composed of the constituents of the blood it- 

 self, and has not to be transformed into the material like the 

 grains and grasses forming the natural diet of the sheep. 



So to comply with these extra requirements the digestive 

 organs of the sheep have to be more spacious to afford means to 

 retain the food until the nourishment can be extracted there- 

 from. To perfom these extra functions the digestive track of 

 herbivora is endowed with greater nervous energy, causing a 

 larger amount of vital and chemical force to be manufactured, 

 thus enabling a greater digestive force to be exerted. 



We find that herbivorous animals in a state of nature are 

 almost continually feeding, a greater amount of the crude ma- 

 terial having to be taken into the system to support life, so to 

 enable the animal to retain this food, the alimentary canal has 

 to be of enormous bulk, digestion being almost continuallv car- 

 ried on. Sheep when grazing tear the grass oif by a jerking 

 action of the muscles of the head and neck, the grass being held 



