CONTRACTILE TISSUES 459 



Monotremes, as investigated by C. J. Martin (1901). Echidna is the lowest 

 member of the scale of warm-blooded animals. If the external temperature 

 changes from 5 to 35, its temperature rises by 10. In cold weather, it 

 hibernates and its temperature is only half a degree above that of its sur- 

 roundings. What regulation it possesses appears to be by change of production 

 of heat. It possesses no sweat glands and exhibits no power of varying loss of 

 heat by cutaneous vasomotor effects, nor does it increase its respiratory 

 movements at high temperatures. The normal temperature of both Echidna 

 and of Ornithorhynchus is 29 -8. In the latter, the temperature is maintained 

 fairly constant, although low. It can modify both heat loss and heat production, 

 but does not increase its respirations at high temperatures. Marsupials show a 

 transition to higher mammals. Variation in production of heat is the ancestral 

 method of adjustment ; by this means an animal combats fall of temperature. 

 Later, a mechanism controlling loss of heat is developed, and thus rise of external 

 temperature is compensated for, as well as the heat produced by the animal's own 

 activity. 



For further details, with regard to production and regulation of temperature, 

 the article by Tigerstedt (1910) may be consulted. 



RHYTHMIC CONTRACTION 



Many organs consisting of smooth muscle, and some with cross-striated 

 muscle, such as the heart, exhibit, even when isolated from the influence of nerve 

 centres, a continued series of periodic contractions and relaxations. From the 

 facts detailed in the preceding pages, it is easy to see how a continuous stimulation 

 might give rise to rhythmic contractions, owing to the refractory period. 



Thus the ventricle of the frog's heart can be excited to rhythmic contraction by a constant 

 current from a battery or by increase of intraventricular pressure. It may be supposed that 

 the first application of the stimulus sets off a beat, but, for a time, the muscle is then inexcitable 

 and, although the stimulus continues, it i8 ineffective. After the refractory period is past, the 

 stimulus again becomes effective and excites a new beat and so on. It seems that the return 

 of excitability has the same effect as a first closure of the current. Apparently, then, a 

 constant stimulus is capable of accounting for rhythmic beats. 



Another possibility to be taken into account is the using up of a store of excit- 

 able material, which has to be replaced and, when accumulated to a certain degree, 

 discharges spontaneously. But certain objections may be made to this view. 



The manner in which rhythmical effects may arise by means of a nerve network may be 

 read in the essay by von Uexkiill (1904). 



The production of rhythmic movements by discharges from the nervous system to skeletal 

 muscles will be discussed in Chapter XVI. 



MOVEMENTS OF PLANTS 



We have seen already how changes of permeability give rise to rapid move- 

 ments in plants by allowing escape of liquid from turgid cells. 



The majority of the usual movements of plants in response to light, gravity, 

 etc., although initiated by changes of permeability, are fixed in their results by 

 different rates of growth on the opposite sides of the moving parts. It is stated 

 that movements due to growth, such as those of tendrils, are considerably more 

 rapid than might be supposed, being detectable in a minute or two. The first 

 stage of many movements is an osmotic one, as remarked, due to changes of 

 permeability and, hence, of turgor. In such a stage, if placed in strong saline 

 solutions, which abolish the turgor on both sides, the curvature is done away 

 with. At a later stage, the curvature is permanent and due to growth. 



There is a point of resemblance between the mechanism of plant and animal 

 movements, otherwise apparently so different, to which attention may be called. 

 The immediate source of the energy of the movement is, in both cases, surface and 

 osmotic energy, although, of course, the ultimate one in the green plant is the 

 sun's radiation and, in the animal, oxidation of material derived from plant life. 

 In the last resort, the animal's energy is also derived from the sun. 



