ECONOMY OF THE BODY 113 



the neuro-muscular. As some would say, the " autonomic " 

 alimentary system rules the " projicient " neuro-muscular 

 system. The demands of hunger compel the bird to hunt, 

 and modern psycho-physiological work has shown that this 

 is a far-reaching fact. The apparent slave is often the real 

 master. 



§ 2. Muscular Activity 



All movements in higher animals are due to muscles, 

 whether locomotion when one piece of skeleton is pulled 

 nearer another, or the beating of the heart when a cavity is 

 essened.by the contraction of its walls. In slowly moving 

 parts of the body, notably the food-canal, the muscle-cells 

 are " smooth " or non-striated — small spindle-shaped cells, 

 dovetailed into one another and united by cement sub- 

 stance. In the \\^[\ of the food-canal they cause peristalsis ; 

 that is to say, their slow contraction gradually presses the 

 food onwards. When there is a violent action, as when the 

 frightened petrel squirts out oil from its mouth, the muscles 

 used are outside the wall of the food-canal, and these are 

 striped and quickly contracting. In backboned animals 

 almost all the muscles, except those in the wall of the gut, 

 the wall of the bladder, and the walls of the arteries, are of 

 the striped or striated type, and the bird is unsurpassable 

 in its muscularity. 



The biological note here is of some interest. The most 

 primitive muscle-cells, seen around the exhalant pores of 

 sponges, are spindle-shaped unstriped cells. These are 

 common in many sluggish backboneless animals, such as 

 tapeworms and oysters. They are well represented in the 

 border-line animals, the Ascidians or Sea-Squirts. In the 

 very interesting annectent type Peripatus, which does some- 

 thing to link insects back to Annelid Worms, all the muscles 

 are unstriped save the quickly moving muscles of the 

 mandibles. Now the point is this, that in backboned 

 animals the primitive slowly contracting type of muscle-cell 

 lingers in such places as the wall of the food-canal, precisely 

 in places where slowness of contraction is useful. 



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