DEGLUTITION AND DEFECATION. 145 



merely breaking the circle of nervous action, through which the movements 

 of Respiration are effected. It is an interesting circumstance, however, which 

 shows the provision made in the animal frame to meet its necessities, that a 

 very small portion only of the nervous centres is involved in this action ; and 

 that, even in the highest Animals, all the rest may be removed, or may be 

 rendered functionally inactive, without checking it. This fact, which was 

 ascertained by Legallois, harmonizes well with that which Comparative 

 Anatomy has brought under our notice ; for it has been shown that, in the 

 lowest groups of Mollusca, but a single ganglion exists ; and that this is 

 almost exclusively concerned in regulating the entrance and egress of the 

 currents of water, the most constant office of which is the aeration of the 

 blood ( 133). 



X. Deglutition, Defecation, 8fc. 



191. Another very important function of the Spinal Cord (and of the 

 ganglia corresponding to it in the Invertebrata), is the control which it exer- 

 cises over the entrance and termination of the Alimentary Canal ; and this 

 reflex action might probably be traced in some animals, in which the neces- 

 sity for that of Respiration does not exist. In all beings which are une- 

 quivocally of an animal character, a stomach or digestive cavity exists ; and 

 a means must be provided for the introduction of food into it. This is partly 

 accomplished by the power with which its entrance is endowed, of contracting- 

 upon, and of attempting to draw inwards, whatever comes in contact with it ; 

 as we may readily observe in the Star-Fish, or Sea-Anemone, where the 

 mouth is simply the aperture of the stomach. From the analogy of the higher 

 animals, as well as from what has been observed in the lower, it seems pro- 

 bable that this action is of a reflex character, depending upon an impression 

 conveyed to the nervous centres, and reflected back to the muscular fibres. 

 But we almost always find some more special apparatus than this for bring- 

 ing food within reach of the orifice of the stomach. In the Sea-Anemone, the, 

 Hydra, and other Polypes, for example, we find that aperture surrounded by 

 tentacula ; which have an evident tendency to lay hold of any thing that 

 touches them, so as to bring it, by their contraction, within reach of tjjeJfctrscles 

 immediately surrounding the orifice. This is just the purpose of ^IsHfearyn- 

 geal muscles of Man. The lower part of the oesophagus, near its te^ilhation 

 in the stomach, has the same simple tendency to contraction from above down- 

 wards (so as to convey into the stomach any thing which is brought within its 

 reach) as have the muscles surrounding the mouth of the Polype ; but there 

 is need of some more complex apparatus, for the purpose of laying hold of the 

 food, and of conducting it into its grasp. This is provided for, in the higher 

 animals, in the muscles of that funnel-like entrance to the oesophagus, which 

 is called the Pharynx. The actions of these are most distinctly reflex ; and it 

 is interesting to remark, that the movements can neither be caused nor con- 

 trolled by the direct influence of the will. In the case of the movements of 

 respiration, we found sufficient provision made for their constant maintenance ; 

 and yet, for secondary purposes, they were placed in a considerable degree 

 under the control of the brain. But here there are no secondary purposes to 

 be answered ; the introduction into the stomach of food, brought by the will 

 within reach of the pharyngeal muscles, is the only object contemplated by 

 them ; and they are accordingly placed under the sole government of the 

 Spinal Cord. No attempts, on our own part, will succeed in producing a 

 really voluntary act of deglutition. In order to excite it, we must supply some 

 stimulus to the fauces. A very small particle of sojid matter, or a little fluid, 

 (saliva, for instance,) or the contact of the back of the tongue itself, will be 

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