The fonnation of the component parts of this system in the embryo has been minutely investigated in various classes of animals, and particularly in the human species. Although there is some difference of opinion as to the exact order in which the various parts make their appearance, it seems to be ascertained that the ganglions of the great sympathetic are formed before the spinal cord and brain; that the nerves on the sides of the trunk, of the head, and of the pelvis, exit independently of the spinal marrow, and indeed have acquired a considerable degree of development, whilst that body and the brain arc still liquid and without form. Next in order, the spinal cord is formed, and lastly, the brain appears.
<Callout type="important" title="Important">The first distinct traces of the corno-spinal axis cannot be perceived in the human embryo until about the fifth week after conception.</Callout>
In the third month, the spinal cord is enlarged opposite the attachment of the limbs, and the rudiments of several parts of the cerebrum and cerebellum make their appearance. Some very important facts have been noticed in this month: the fibres of the spinal cord may be seen radiating upon the internal surface of the hemispheres; the latter are observed to curve from before backwards, and from without inwards, so as to form the lateral ventricles; they also unite in the median line, and thus give rise to the corpus callosum.
<Callout type="risk" title="Risk">During the sixth month, the pyramidal, olivary, and restiform bodies are considerably enlarged; the latter penetrate into the cerebellum; the fibres of the corpora pyramidalia et olivaria traverse the annular protuberance, and then constitute the peduncles of the cerebrum.</Callout>
At the seventh month, the lateral lobes of the cerebellum are proportionally more developed than the central part, which appears rather sunk or depressed. The transverse and ascending fibres of the annular protuberance are distinctly observed interlacing each other.
The various parts of the encephalon and spinal cord have acquired by the eighth month their elementary form that the remaining changes which they undergo till they attain their perfect organization relate to increase in volume and density. The canal of the spinal cord is nearly filled with a nutty grey substance, which appears to be deposited from the vessels of the pia mater that enter by the posterior furrow. The fibres of the bodies placed on the medulla oblongata are more distinctly observed passing to the respective organs with which they are connected.
<Callout type="tip" title="Tip">The increase of the nervous system, which is so rapid during the fetal life, is checked after birth; the growth of the various parts, however, still proceeds, but more slowly.</Callout>
It is remarkable that during the whole period of the fatal existence the spinal marrow appears gradually to ascend in the cerebral canal. When it is first examined, it reaches to the extremity of the os coccygis, and thus resembles the caudal prolongation of animals; subsequently, the termination is observed to be placed higher and higher, till at the time of birth it is situated opposite to the upper part of the third vertebra of the lumbus.
The expression of cerebro-spinal system was employed by M. Desmoulins to designate that part of the nervous system which consists of the brain and spinal cord. It appears to me that its meaning may be appropriately extended so as to include not only the encephalon and spinal marrow, but also the nerves which arise from those organs. With this signification, the term corresponds with 'the nervous system of animal life,' suggested by Bichat.
Key Takeaways
- The spinal cord and brain develop in a specific order, with the sympathetic ganglia forming first.
- The corpus callosum develops from the union of fibres in the third month of fetal life.
- The cerebellum shows significant growth by the seventh month.
Practical Tips
- Understand that the nervous system's development is a complex process with distinct stages, which can help predict and manage neurological issues in infants.
- Recognize the importance of proper prenatal care to support healthy fetal brain and spinal cord development.
- Be aware of the potential risks during the sixth month when significant changes occur in the formation of key brain structures.
Warnings & Risks
Warning
During the eighth month, the spinal cord is nearly filled with a grey substance that may affect its function if disrupted.
- Be cautious about applying this knowledge to modern survival scenarios without proper medical guidance, as techniques and understanding have advanced significantly.
Modern Application
While the detailed embryological processes described in this chapter are fascinating, much of our current understanding has expanded beyond these historical observations. However, the principles of fetal development remain crucial for prenatal care and neonatal health. This knowledge still matters because it forms the basis for modern obstetric practices and can inform emergency response to cases involving pregnant individuals or newborns.
Frequently Asked Questions
Q: What is the order in which parts of the nervous system develop?
According to the text, the ganglions of the great sympathetic are formed before the spinal cord and brain. The spinal cord develops next, followed by the appearance of the brain.
Q: How does the development of the corpus callosum occur?
The corpus callosum arises from the union of fibres that unite in the median line during the third month of fetal life.
Q: What changes occur to the spinal cord by the eighth month?
By the eighth month, the canal of the spinal cord is nearly filled with a grey substance, and the fibres of the bodies placed on the medulla oblongata are more distinctly observed passing to their respective organs.