GENERAL VIEW OF THE PROCESS OF NUTRITION. 593 



of the blood through the extreme vessels.* This view has recently obtained 

 additional support from the experiments of Dr. J. Reid on the Respiration of 

 Azote.t He found that, when the ordinary respiration of an animal is inter- 

 rupted, and the Asphyxia is proceeding to the stage of insensibility, the first 

 effect upon the arterial system is an increased distension (as indicated by the 

 hsemadynamometer), even although the blood is at that time nearly venous in 

 its character ; this indicates that the fluid, now so perverted, is unable to pass 

 with facility through the systemic capillaries, in consequence of its not being 

 in a state fit for the performance of its normal actions. As the stagnation in 

 the pulmonary capillaries becomes more complete, however, less and less 

 blood is returned from the lungs to the heart; and, the systemic arteries 

 being gradually unloaded without being refilled, the pressure of the blood 

 upon their walls diminishes, and is at last no longer experienced. Its dimi- 

 nution is not arrested by causing the animal to breathe nitrogen, although the 

 respiratory movements are renewed, thus proving that the stagnation of the 

 blood in the capillaries of the lungs is not due (as some have supposed) to a 

 mechanical impediment: but the pressure is immediately increased by the 

 admission of atmospheric air, which occasions the renewal of the pulmonary 

 circulation, and the consequent increase in the supply of aerated blood to the 

 systemic arteries. It has been shown by Mr. Wharton Jones,J that the ca- 

 pillary circulation in a frog's foot is retarded or even checked, by the direc- 

 tion of a stream of carbonic acid gas against the membrane ; and he attributes 

 this stagnation to the disposition thus produced in the red corpuscles, to ag- 

 gregate together and to adhere to the walls of the vessel, so as to choke up 

 its calibre. 



CHAPTER XIV. 



OF NUTRITION. 



1. General Considerations. Selective Power of Individual Parts. 



781. THE function of Nutrition, considered in the widest acceptation of the 

 term, includes the whole series of processes, by which the fluid alimentary 

 materials, prepared by the Digestive process, introduced into the system by 

 Absorption, and carried into its penetralia by the Circulation, are converted 

 into Organized tissue ; by which conversion it is caused to manifest a set of 

 properties altogether new, which, being neither Physical nor Chemical, are 

 termed Vital. Thus Albumen, which is a perfectly dead or inert substance, 

 and of which the distinguishing properties are entirely attributable to its pecu- 

 liar composition, is transformed by the Nutritive process into Muscular Fibre, 

 possessed of the remarkable Vital property of Contractility. But this process 

 of conversion commences in the nutritive materials whilst they are still in a 

 fluid condition, and are moving through the vessels ; for we have seen that, at 

 this stage of the operation, the unorganizable Albumen is transformed into 



' For a fuller discussion of the Pathology of Asphyxia, see the Author's essay on the sub- 

 ject in the Library of Practical Medicine, vol. iii. 

 t Edinb. Med. and Surg. Journal, April 1841. 

 j British and Foreign Medical Review, vol. xiv., p. 600. 



50* 



