344 OF NUTRITION. 



614. The selecting power, which is possessed by the germs of each 

 kind of tissue, and which enables them to draw from the blood the 

 materials which they severally require for their development, manifests 

 itself also in the mode in which substances, that are abnormally present 

 in the blood, affect the condition and development of the solid tissues. 

 Thus we find that the presence of a certain quantity of Arsenic in the 

 blood, -will produce a state of irritation in all the Mucous membranes of 

 the body. The continued introduction of Lead into the circulating 

 system, occasions a modification in the nutrition of the extensor muscles 

 of the fore-arm, producing the form of partial paralysis commonly 

 termed "wrist-drop," and the existence of this modification is shown 

 by the presence of lead in the palsied muscles. Here we have to 

 remark the symmetrical nature of the affection, consequent upon the 

 occurrence of the same disorder in the corresponding parts of the two 

 sides of the body ; for these muscles appear to have the same kind of 

 tendency to attract lead from the circulating current, in a degree that 

 is equal on the two sides, as they have to draw from the blood the 

 materials of their regular growth, and to develope themselves in an 

 exactly similar manner. In like manner, the cutaneous eruptions, which 

 are occasionally produced by the internal exhibition of iodide of po- 

 tassium, are found to be almost precisely symmetrical : the presence of 

 the medicine in the blood being the occasion of a disordered nutrition 

 of certain parts of the skin ; and the selecting power of particular spots 

 being evinced, by the exact correspondence of the parts affected on the 

 two sides. 



615. The same appears to be the case with regard to substances, 

 whose presence in the blood is rather the result of a disordered condi- 

 tion of the digestive and assimilating processes, than of their direct 

 introduction from without. Thus in Lepra and Psoriasis, chronic 

 diseases of the Skin, which seem to have their origin in a disordered 

 state of the blood, rather than in the solid tissues affected, we find a 

 remarkable tendency to the repetition of the patches on the two sides 

 of the body, or on the corresponding parts of the limbs ; and this we 

 must attribute to the peculiar attraction, existing between the solid 

 tissues of those parts, and the morbid matter circulating through them. 

 So in those chronic forms of Gout and Kheumatism, which modify the 

 nutrition of the joints, producing a deposit of " chalk-stones," or per- 

 manent distortion and stiffening, we almost invariably find the corre- 

 sponding joints of the two sides affected. The chief exceptions to the 

 general principle, that the presence of morbid or extraneous matters in 

 the blood affects corresponding parts alike, are found to exist where 

 there is much febrile disturbance, or where local causes produce a 

 peculiar tendency to disorder of a single part. The nearer- the character 

 of the morbid process is to that of the ordinary nutritive operations, the 

 more nearly does it approach these, in the symmetry with which it 

 developes itself.* 



* See Dr. W. Budd's valuable Paper on the " Symmetry of Disease," in vol. xxv. of 

 the Medico-Chirurgical Transactions. 



