270 



DESIGN IN NATURE 



The following table furnishes some interesting information regarding the periods of gestation m mammals 

 PERIODS OF UTERO-GESTATION IN ANIMALS 



The alimentary canal receives, retains, and passes on the food in relays systematically and at stated intervals ; 

 the chest draws the air into the lungs, and keeps it there, for a sufficiently long period to admit of the aeration of the 

 blood, after which it is expelled ; the bladder collects and retains the urine for variable periods without discomfort ; 

 the uterus receives the ovum, which develops into the embryo and foetus, and retains them for periods varying from 

 a few weeks up to over a year. Lastly, the several compartments of the heart receive and retain their respective 

 quotients of blood, which are duly discharged either into other compartments or into the great blood-vessels 

 (pulmonary artery and aorta). The several compartments of the heart do not discharge the blood which flows 

 into them the instant it enters. On the contrary, the blood is allowed to collect, and, when a full complement has 

 been received, and not till then, is it discharged. It is, moreover, not discharged until a free space is provided for 

 its reception. All this means prevision and design. When the auricles close and perform their centripetal expelling 

 movements, the ventricles open and perform their centrifugal sucking movements : when the ventricles have drawn 

 full measures of blood into their interior by their centrifugal movements, they close and perform their centripetal 

 expelUng movements. The centripetal and centrifugal movements of the auricles and ventricles of the heart are 

 vital in their nature. 



The compartments of the heart of the bird and mammal act in pairs, and provide vis a fronte and vis a tergo move- 

 ments ; the auricles opening, sohciting, and sucking in blood when the ventricles close, discharge, and expel it. The 

 closing movements of the auricles do not occasion the opening movements of the ventricles, or vice versa. The 

 auricles have no power to open the ventricles, or the ventricles the auricles. Each compartment of the heart acts 

 spontaneously and independently, but the movements of the several compartments are co-ordinated, and all act 

 to a given end, namely, the reception, retention, and propulsion of the blood. 



In the movements of the heart the elastic properties of the cardiac muscles and the elastic recoil of the great 

 vessels play quite a subordinate part. The muscles of the heart are the prime movers of the blood. The blood 

 cannot cause its own expulsion from one compartment of the heart into another : neither can it cause its own 

 transmission through the arteries, veins, and capillaries. As regards the movements of the heart in the adult, 

 they go on when the organ is cut out of the body, deprived of its blood, and placed in an exhausted receiver — that 

 is, when the heart is deprived of blood and air. This gets rid of inherent irritability and extraneous stimulation 

 as factors in the movements of the heart. That the presence and pressure of blood is not necessary to the move- 

 ments of the heart is proved by this, that its several compartments open and close and beat in young animals before 

 they contain blood. Of late years many are inclined to beheve that the movements of the heart are due wholly or 

 in part to rhythmic nerve action. This view is negatived by the fact that the heart displays rhythmic movements 

 before nerves are developed in it, and even before it contains muscular fibres. A living muscular fibre, moreover, 

 shortens and elongates when deprived of its nervous supply ; all which goes to prove that the movements of the 

 rhythmic muscles are inherent, fundamental, and spontaneous. 



While it would be obviously incorrect to deny an inherent power of moving to rhj'thmic muscles, it would be 

 equally incorrect to say that nerve cells, ganglia, and nerves cannot act rhythmically. The very intimate connection 

 which obtains between muscles and nerves forbids such a conclusion. The force of habit alone, in nerves supphed 

 to rhythmic muscles, would, there is reason to beheve, sooner or later, result in rhythmic or interrupted nerve action. 



