502 OF RESPIRATION. 



It is maintained by Prof. Kblliker that the office of the muscular fibres which 

 pass in every direction amongst the dilated veins, is to keep them compressed 

 in the intervals of erection, so as to prevent them from being distended by the 

 vis a tergo of the blood ; and that the stimulus to erection, which is usually 

 conveyed through the nervous system, so operates upon these fibres as to occa- 

 sion their relaxation, whereby the free distension of the cavernous veins and of 

 the arterial diverticula is permitted. He refers, moreover, to the excessive con- 

 traction of erectile organs which is induced by cold, and to the effect of warmth- 

 in favoring their enlargement, as confirmatory of this view; and considers that 

 no other agency is required. 1 



CHAPTER X. 



OF RESPIRATION. 



1. Nature of the Function : and Provisions for its Performance. 



535. THE Nutritive fluid, in its circulation through the capillaries of the 

 system, undergoes great alterations, both in its physical constitution and in its 

 vital properties. It gives up to the tissues with which it is brought into con- 

 tact, some of its most important elements ; and at the same time, it is made the 

 vehicle of the removal, from these tissues, of ingredients which are no longer 

 in the state of combination that fits them for their offices in the Animal Economy. 

 To separate these ingredients from the general current of the circulation, and 

 to carry them out of the system, is the great object of the Excretory organs; 

 and it is very evident that the importance of their respective functions will vary 

 with the amount of the ingredient which they have to separate, and with the 

 deleterious influence which its retention would exert on the welfare of the sys- 

 tem at large. Of all these injurious ingredients, Carbonic Acid is without 

 doubt the one most abundantly introduced into the nutritive fluid ; and it is also 

 most deleterious in its effects on the system, if allowed to accumulate. We find, 

 accordingly, that the provision for the removal of this substance from the blood, 

 is one of peculiar extent and importance, especially in the higher forms of 

 animals; and further, that instead of being effected by an operation peculiarly 

 vital (like other acts of Excretion), its performance is secured by being made 

 to depend upon simple physical conditions, and is thus comparatively little sus- 

 ceptible of derangement from disorder of other processes. All that is requisite 

 for it, is the exposure of the Blood to the influence of the Atmospheric air (or, 

 in aquatic animals, of air dissolved in water), through the medium of a mem- 

 brane that shall permit the " diffusion of gases;" an interchange then taking 

 place between the gaseous matters on the two sides Carbonic acid being exhaled 

 from the Blood, and being replaced by Oxygen from the air. Thus the extri- 

 cation of Carbonic acid is effected in a manner that renders it subservient to the 

 introduction of that element which is required for all the most active manifesta- 

 tions of vital power ; and it is in these two processes conjointly, not in either 

 alone, that the function of Respiration essentially consists. We shall now 



1 See his essay "Das Anatomisclie und Physiologische Verhalten der Cavernoser Korper 

 der Sexual organe," 1851 ; cited in Paget and Kirkes's " Handbook of Physiology," 2d 

 edit., p. 145. It is rather difficult to admit the power of nerves to cause relaxation of 

 muscles, which this hypothesis requires ; and also to explain in accordance with it the fact 

 of very familiar occurrence, that the application of moderate cold (as in putting on a clean 

 shirt) frequently occasions erection of the male nipple. 



