CHANGES OF FOOD IN THE ALIMENTARY CANAL. 411 



especially the case, when their power is augmented by increased nutrition. For 

 example, in many cases of disease or injury of the Spinal Cord, the bladder 

 ceases to expel its contents, through the interruption of the circle of reflex 

 actions; but after a time, the necessity for drawing off the urine by the catheter 

 is found to exist no longer, the fluid being constantly expelled as soon as it has 

 accumulated in small quantities. In such cases, the mucous coat is found after 

 death to be thickened and inflamed; and the muscular coat to be greatly in- 

 creased in strength, and contracted upon itself. It would seem, then, that the 

 abnormal irritability of the mucous membrane, and the increased nutrition of 

 the muscular substance which appears consequent upon it, enable the latter to 

 expel the urine without the assistance of the ordinary expulsors. 



435. On the other hand, the sphincters which antagonize the expellent action, 

 are usually maintained in a state of moderate contraction, so as to afford a con- 

 stant check to the egress of the contents of the cavities; and this condition has 

 been fully proved by Dr. M. Hall, to result from their connection with the 

 Spinal Cord, ceasing completely when this is interrupted. But the sphincters 

 are certainly in part controlled by the will, and are made to act in obedience to 

 the warning given by sensation; and this voluntary power is frequently destroyed 

 by injuries of the Brain, whilst the Spinal Cord remains able to perform all its 

 own functions, so that discharge of the urine and faeces occurs. In their moderate 

 action, the expulsors and the sphincters may be regarded as balancing one 

 another, so far as their reflex action is concerned; the latter having rather the 

 predominance, so as to restrain the operation of the former. But, when the 

 quantity or quality of the contents of the cavity gives an excessive stimulus to 

 the former, their action predominates, unless the will be put in force to strengthen 

 the resistance of the sphincter; this we are frequently experiencing, sometimes 

 to our great discomfort. On the other hand, if the stimulus be deficient, the 

 will must aid the expulsors, in order to overcome that resistance which is due 

 to the reflex contraction of the sphincters; of this also we may convince our- 

 selves, when a sense of propriety, or a prospective regard to convenience, occa- 

 sions us to evacuate the contents of the rectum or bladder without a natural 

 call to do so. 



4. Of the Changes which the Food undergoes, during its passage along the 



Alimentary Canal. 



436. The object of the Digestive process, as already pointed out, is to reduce 

 the Alimentary matters to a condition in which they can be introduced by 

 Absorption into the Circulating system. This reduc- 

 tion is partly effected, as we have seen, by Mechanical Fig. 121. 

 means ; but it is chiefly due to the chemical agencies 



which are brought to bear upon the ingested sub- 

 stances during their transit through the mouth, the 

 stomach, and the upper portion of the intestinal 

 tube. The first of these is exerted by the Salivary 

 fluid, which is incorporated with the food in the act 

 of mastication, and of which a large quantity de- 

 scends with it to the stomach. For the secretion of 

 this fluid, it will be remembered that three pairs of 

 glands of considerable size are provided; namely, the 

 parotid, the sublingual, and the submaxillary. But 

 in addition to these, a very important part of the 

 fluid is furnished by the numerous follicular glands 

 lodged in and beneath the buccal mucous membrane. L ^ ule of Par m Gland of a 



rru a r i j new-born Infant, injected with 



The Salivary glands are constructed upon that folh- mercury . Magnified 50 diameters. 



