THE DIGESTIVE AND RESPIRATORY SYSTEM 291 



partly under the control of the will, extend farther down, and 

 in ruminants, where the food is voluntarily disgorged in the 

 form of cuds for a second chewing, the entire oesophagus is 

 thus equipped. 



The stomach is originally a simple, spindle-shaped enlarge- 

 ment of the canal, extended lengthwise and indefinitely sepa- 

 rated from the oesophagus, although more completely limited 

 below by a restriction, the pylorus, which forms a valve for 

 the purpose of temporarily converting it into a closed sac. 

 This typical form is seen in fishes and tailed amphibians, but 

 an attempt to increase its efficiency both in digestive surface 

 and in capacity causes the formation of a curvature extending 

 to the left, so that there is a longer outline upon its left side 

 and a shorter one upon its right, the greater and lesser curva- 

 tures respectively. As the same tendency continues the 

 stomach turns, still to the left, in such a way that ultimately 

 its longitudinal axis lies across the body, which places the 

 upper or cardiac end on the left, the lower or pyloric end, on 

 the right, the lesser curvature above and the greater curvature 

 below. Below the cardiac orifice the left end of the stomach 

 usually bulges out laterally to form the fundus, which in some 

 cases becomes a more or less distinct receptacle for the food 

 when first received ; and beyond approximately the middle the 

 stomach tapers toward the pyloric end (right) and forms an 

 upward curve at the culmination of which is placed the pylo- 

 rus. This may be considered the typical form of mammalian 

 stomach and is seen in Primates, Carnivora, Insectivora, and 

 Edentata, these being in other respects also most primitive 

 of placental mammals (Fig. 80, a). 



Modifications of this primary form are due, first, to an at- 

 tempt to localize and define the different portions of the stom- 

 ach and specialize their functions, and, secondly, to various 

 attempts to increase the general surface and thus develop a 

 greater physiological efficiency, usually in connection with in- 

 nutritious food or with the necessity of taking in a large 

 amount in a short space of time. 



Progress in the first of these directions is shown by such a 



