8 Nov., 1907.] Elements of Animal PJiysiologv. 691 



external sphincter of the anus is remarkable in that its muscle is striped 

 or voluntary. 



The acts of prehension and mastication (chewing) of food are under 

 the will, and are carried cut 1\\ slightix different methcds in difterent 

 species. The act of swallowing is usually divided inttj three stages. 

 In the first, which is \oluntary, the food, after mastication, is passed 

 to tlae back of the moiith and into the pharynx. Once the food is in 

 the pharynx the second stage takes place reflexlv and independent of 

 the will. A strong peristalsis of the pharynx occurs and the root of the 

 tongue is jerked against the retreating bolus of food. At the same time 

 the hinder openings of the nostrils are closed by the soft palate, and 

 the entrance to the wind-pipe is closed by this being drawn forward under 

 the tongue. By the vigorous muscular action of the pharvnx and of 

 the tongue the food is shot into the oesophagus where the third stage 

 takes place. In this the food is carried towards the stomach or crop by 

 a quick and involuntary peristalsis and as the food arrives at the termination 

 of the gullet the sphincter guarding the entrance to the stomach or crop 

 relaxes, lets the food pass, and then closes again. The muscular character 

 of peristalsis explains why most animals are able to eat and drink with 

 their heads lower than their stomachs. 



Vomiting can take place readily in carnivores and omnivores but in 

 solipeds it is of very rare occurrence and is almost unknown in ruminants. 

 When a man vomits a deep breath of air is taken and the esca])e of the 

 air prevented by closure of the glottis. Then the abdominal and thoracic 

 muscles contract vigorously — an action which, if the glottis were not 

 closed, would only drive air out of the lung, but owing to the closure 

 of the glottis the pressure is borne by the abdominal organs. The 

 muscles of the stomach contract too and the sphincter between the 

 oesophagus and the stomach is relaxed. The pressure on the stomach 

 contents forces them along the oesophagus and so into die mouth (nostril 

 of the horse). Vomiting is a reflex generallv started I>v the presence of 

 irritating matter in the stomach, though it ma\- also' l)e due to nervous 

 diseases and disturbances. 



Defascation is the act of extrusion of faeces through the anus. When 

 the fsecal mass accumulates in the lower colon it excites reflexlv a strong 

 peristalsis by which it is carried through the rectum. The anal sphincter 

 then relaxes and a strong contraction of the rectum drives the faecal 

 mass outwards. This action is generalh- helped by the abdominal 

 muscles contracting and exercising a pressure as in vomiting. Rumination 

 will be discussed under digestion in the stomach. 



BLOOD SUPPLY OF THE ALIMENTARY CANAL.— All parts 

 of the alimentary tube are lichly supplied with blood vessels that bring 

 red, arterial blood from the aorta. A remarkable peculiaritv is present 

 in the parts of the tube that lie in the abdominal region. The blood 

 which has traversed these portions, together with that from the pancreas 

 and spleen, instead of passing into veins that flow direct to the heart, 

 is collected by a sxstem of veins which join into one great vein called 

 the PORTAL VEIN. This portal vein runs towards the liver and breaks 

 up into myriads of branches in that organ so that the blood is brought 

 into the sphere of action of the liver cells. Then from the liver it passes 

 by the ordinary route of the veins to the heart. Thus all the l:)lood 

 drained from the abdominal digestive organs and from the spleen must 

 pass through the liver before it can reach the general circulation. Tlie 

 great importance of this we 'shall see later. 



