538 TRANSACTIONS OF SECTION I. 



number directly equivalent to this demand, that the blood should be capable of 

 transferring a certain quantity of oxygen. Thus if these structures are lost by 

 haemorrhage, or rendered less efficient by the presence of carbon monoxide, or 

 when circumstances for the acquisition of oxygen are peculiarly difficult, as on 

 high altitudes, their formation is proportionally accelerated. That negative 

 pressure of oxygen governs blood-production is a statement which will bear some 

 inspection. 



Now here we have a function which for its perfect performance is dependent 

 upon another machine, the respiratory mechanism, which in its turn is governed 

 by a different but correlated factor — namely, the carbonic-acid pressure in the 

 blood. In this case we may say that the positive pressure of carbonic acid 

 dominates the quantity of the respiratory activity. It is well known now that 

 this statement has been set on firm ground. 



It is interesting, then, to observe how these two mechanisms are brought into 

 exact correlation by the simple fact that the lung surface, a portion of the 

 respiratory mechanism, is formed accurately to a measure provided by the volume 

 of blood despatched from the heart, and therefore probably by that second 

 growth of blood-tissue which I have spoken of as due to pressure from the 

 heart. The surface of the lungs is some eighty square metres. The heart at 

 each stroke sends into the lungs somewhere about 100 cubic centimetres of blood 

 containing red corpuscles within a total surface also of 80 square metres. Here, 

 then, we have a mechanical link connecting these mechanisms that is obviously 

 forged by an incident of use. 



Within the central nervous system where development mainly affects the 

 shape and distribution of structures rather than their chemical quality, affecting 

 thus what we might call the geography of the system, interesting geographical 

 facts attest to the same forged linkage of mechanisms. Thus, for example, we 

 have the so-called ' sympathetic system,' offering at first view a curious anomaly 

 to the more usual somewhat segmental distribution of nerve-fibres, since from 

 the region of the cord related to the trunk of the body nerves pass through 

 this system to control tissues placed in the head and in the limbs. This 

 anomalous geographical fact is. however, at once explained when we regard the 

 part played by this sympathetic control in the several parts of the body as 

 merely subservient to the interests of locomotion. Under its influence the eye 

 is set for out-of-door, or, if I might say it, for out-of-cave vision. The heart 

 is accelerated. The glandular organs, with the exception of those useful in times 

 of much exertion and heat production, like the sweat glands, are set at rest or 

 else the motor organs of special importance in their sphere of influence are 

 quietened. Regarding the matter in this light there is nn obvious convenience of 

 geographical fact in the situation of this instrument midway between those 

 parts of the central nervous system that are swept at this very time by nervous 

 impulses dominating the movements of the limbs ; just as there is some con- 

 venience in the chemical linkage which has been discovered between the different 

 parts of this sympathetic system that further tends to permit their unison of 

 activity. 



On the other hand, when the muscles are at rest and the condition of the 

 body is of the indoor description, the eye is -set for close vision, and various 

 glandular organs are allowed to conduct their functions under the influence of 

 nervous mechanisms placed at some distance from the distributing centres of 

 nervous activities that are used in locomotion. 



Doubtless this useful distribution of parts within the nervous system must 

 find an explanation in the same terms as must the dynamic anatomical relation 

 to which I have drawn attention as linking up the respiratory and circulatory 

 systems — namely, the fact that the heart sweeps past the surface of the lungs at 

 each stroke red corpuscles that have the same extent of surface ae the lungs. 

 In both cases it is true that the right adjustment of the several parts of this 

 machine has been arrived at as a consequence of use, and that these mechanical 

 linkages are due to circumstances of a purely physical and chemical nature. 



In conclusion I might say that these instances have been selected to illustrate 

 my opinion that some of the experiments of greatest interest to physiology are 

 in process of conduction within the normal body, and are to be observed by 

 records imprinted on its structures. In feeling for the keys whereby each set 

 of records may be interpreted it is necessary that someone should frankly 



