236 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



with the single doubtful exception of the dog, are unable to close 

 the mouth. 



2. Prehension of Liquids. — In the lower forms of animal life pre- 

 hension of liquid is accomplished by absorption through the general 

 external body surface, and in many cases, as, for instance, in the tape- 

 worm, where neither mouth nor stomach are present, the fluids so 

 absorbed carry also the nutritive matters in solution into its interior. 

 Many other animals which live on liquid food are provided with special 

 organs for absorption ; thus, in the leech there is a mouth or sucker, 

 provided with minute teeth for piercing the skin of other animals, while 

 in the mosquito there is a sharp, bristle-like tube for piercing the skin, 

 and in the louse there is a sharp sucker, armed with barbs to fix it 

 securely during the act of sucking. In certain insects which live on 

 viscid or fluid food, as the butterflies and moths, the mandibular append- 

 ages are modified from their usual form described in the preceding 

 section, and take on the form of a long, spiral tube, the proboscis, which 

 can be unfolded and protruded into flowers. A sucking proboscis also 

 is found in many flies and gnats. In fleas and bugs the mandibles are 

 penetrating and suctorial. In the higher animals no special prehensile 

 organs for the absorption of liquids are present, it being accomplished 

 by means of the apparatus already described for the prehension of 

 solids. Four methods for the prehension of liquids have, however, been 

 described by Colin : — 



a. Suction, as in the drawing of milk by young animals. 



b. Pumping, by the immersion of the lips and the piston-like action 

 of the tongue within the mouth, on the principle of the common pump. 



c. Aspiration, where the vacuum is produced by an inspiratory 

 movement, as well as by the motion of the tongue. 



d. By lapping or ladling the fluid by the tongue into the mouth. 



a. Suction. — In suckling, the teat is grasped by the lips, or, it may 

 be, even by the teeth, and the mouth closed around it. The tongue is 

 then pressed against the teat and withdrawn into the mouth, producing a 

 vacuum, and from the atmospheric pressure on the exterior of the breast 

 the milk then enters the mouth. There is, therefore, no danger of milk 

 entering the windpipe, since inspiration is not at all concerned in the 

 process of suckling ; hence, aquatic animals, like the cetaceans, may 

 suckle under water. In solipedes and ruminants, during suckling, the 

 tip of the tongue is often fixed between the teeth and the nipple ; the 

 vacuum is then made by the reduction in volume of the anterior part of 

 the tongue, while the base becomes applied to the roof of the mouth. 



During the act of suckling, the sterno-thyroid and the sterno-, omo-, 

 and thyro-hyoid muscles contract together, and so depress the larynx 

 and hyoid bone, while at the same time the hyoid bone is advanced by 



