PHYSIOLOGICAL rilEXtiMHXA IX TIfK FCETUS. llt> 



Nervous Functions. 



The developmeut of the tiervous functiojis is one of these phenomena 

 worthy of attention. It appears certain that the young creature is 

 formed and developed without the direct intervention of the nervous 

 intiuence of the parent, as there is no coniinunication between the 

 nerves of the uterus and the fcetal envelopes ; and it has not been 

 shown that there are nerves either in the latter, or yet in the umbilical 

 cord. The progressive animation of the new creature cannot, therefore, 

 be derived from immediate nervous propagation, or direct extension of 

 the nervous activity from the mother to its offspring ; and we must 

 look upon this animation as originating in the embryo, and becoming 

 developed under the influence of inappreciable causes. At the very 

 commencement of embryonic life, the nervous system, which, in extra- 

 uterine existence, holds under its control the organic functions, appears 

 now to have no possible influence on the phenomena that gradually 

 manifest themselves. The primary changes in the ovum occur before 

 the earliest traces of the nervous system are apparent ; and while its 

 rudiments are forming, those of other parts are also being developed. 

 Even when the nervous system has attained a somewhat considerable 

 growth, it does not appear to have assumed those controlling powers 

 with which it is so largely endowed at a subsequent period. 



In the homogeneous plasma of the ovum, in the common blastema, 

 coincidently or successively, a multitude of diilerent parts are being 

 formed independently of each other, as if each contained within itself 

 the why and wherefore of its formation and ulterior perfecting. The 

 nervous system itself seems to be submitted to the same general laws, 

 and it does not appear to be dependent upon any other part, neither does 

 any part depend upon it ; even when it is fully formed, the phenomena 

 pertaining to growth are evidently effected without its stinmlating 

 intervention. Besides, it is well known that the monstrosities which 

 are sometimes seen destitute of brain and spinal cord, may yet reach 

 the end of their foetal development. True, certain facts have appeared 

 to demonstrate that the formation of parts is dependent on nervous 

 action. In certain monstrosities, organs have not become developed 

 when the nerves which should have been distributed to them were 

 absent ; in others, several organs have become fused together when 

 their nerves were similarly aggregated ; while an organ has been 

 divided into fractions corresponding to the accidental development of 

 its nerve. The muscles of the posterior part of the body of the fa'tus of 

 the Cow and Pig have been obsen-ed to be absent when the corre- 

 sponding portion of the spinal cord was, with its nerves, very imper- 

 fectly developed. But these facts do not prove that tlie absence of the 

 nerves was the cause of the non-development of the nmscles ; for in 

 such cases there is a correlation between the non-formation of the 

 nerve and the absence of the muscle; the same cause whicli has hindered 

 the formation of the one in a certain part of the body has also prevented 

 the growth of the other in that part. It is the same, to a certain point, 

 with the relative development of vessels and the increase of the parts to 

 which they are distributed. In the original homogeneous plasma, there 

 were developed the diverse elements of an organ at the same time as 

 its vessels, and the force which created the connective, nervous, and 

 muscular tissues, created also the elements of the vessels — ai'teries, 

 veins, and capillaries ; they are closely related to each other, and their 



