Page:Illustrations of the comparative anatomy of the nervous system.djvu/45

 formed, but the nervous system, as a whole, may still be inferior to that of creatures possessing a spinal marrow; it may, however, have been altogether so much altered in form and qualified in composition, as to be very different from that in man, but suited to correspond with the organization of each animal.

Much very curious and interesting information has been obtained from the examination of the foetus during its different stages of uterine existence, and particularly respecting the resemblance of the brain to that of the different classes of animals according to its approach to or its distance from maturity. In the human feetus, the sympathetic as well as the spinal nerves and their ganglia are formed at an early period, and when very little more is apparent than the membranes of the spinal marrow. This circumstance favours the opinion, that the long chord of invertebrated animals is not the prototype of the spinal marrow, but of the spinal ganglia and their nerves, the sympathetic and par vagum, according to the preceding observations. But, in whatever way the subject be considered, much must be left to conjecture. A very distant resemblance is all that can reasonably be expected to be found in creatures so different in their form and mode of existence.