. 



281 



One thing well worthy of attention, and on which no anatomist has 

 dwelt, is that the brain of the foetus, and of the child just born, appears 

 to consist almost entirely, of a cineritious pulp, to such a degree that the 

 medullary substance is difficult to perceive in it. Would it be absurd to 

 believe, that the medullary part of the brain does not take its perfect or- 

 ganization till after birth, by the developement of the fasciculi of medul- 

 lary fibres, in the midst of these masses of cineritious substance, which 

 must be considered as the common source from which the nerves have 

 their origin, or, to use the language of Gall, as the uterus which gives 

 them birth. The almost total inactivity, the passive state of the brain, 

 in the foetus, makes unnecessary there the existence of the medullary ap- 

 paratus, to which the most important operations of intelligence seem en- 

 trusted. Its first rudiments are found in the foetus at its full time. That 

 fibro medullary apparatus will be strengthened by the exercise of thought, 

 as the muscles are seen to enlarge and perfect their growth, by the effect 

 of muscular action. 



CXLV. Circulation of the brain. I have said that the blood, in its 

 circular course, does not traverse the different parts of the body with uni- 

 form velocity; that there are partial circulations in the midst of the ge- 

 neral circulation. In no organ are the laws, to which this function is 

 subjected, more remarkably modified than in the brain. There is none 

 which receives, in proportion to its bulk, larger arteries and more in num- 

 ber. The internal carotid and vertebral arteries, as we may satisfy our- 

 selves from the calculations ot tialler, carry thither a great portion of the 

 whole quantity of blood that flows along the aorta ; (from a third to the 

 half.) 



The blood which goes to the brain, said Boerhaave, is more aerated 

 than that which is distributed to the other parts : the observation is not 

 without foundation. Though the blood which the contractions of the left 

 ventricle send Into the vessels, arising from the arch of the aorta, does 

 not undergo, at the place of this curvature, a mechanical separation car- 

 rying its lighter parts towards the head; it is not less true, that this 

 blood, just passing from the contact of the air in the lungs, possesses, 

 in the highest degree, all the peculiar qualities of arterial blood. So 

 great a quantity of light, red, frothy blood, impregnated with caloric 

 and oxygen, coming upon the brain, with all the force it has received 

 from the action of the heart, would unavoidably have deranged its soft 

 and delicate structure, if nature had not multiplied precautions to weak- 

 en its impulse. 



The fluid compelled to ascend against its own weight, loses, from that 

 alone, a part of its motion. The vertical column must strike against the 

 angular curvature which the internal carotid takes in its passage along 

 the osseous canal of the petrous portion of the temporal bone, and as this 

 curvature, supported by hard parts, cannot straighten itself, the column 

 of blood is violently broken and turned out of its first direction, with con- 

 siderable loss of velocity. 



The artery immersed in the blood of the cavernous sinus, as it comes 

 out from the carotid canal, is very easily dilated. Finally, the branches 

 into which it parts, on reaching the base of the brain, have coats exceed- 

 ingly thin, and so weak that they collapse, when they are empty, like 

 those of the veins. This weakness of the cerebral arteries, explains their 

 frequent ruptures, when the heart sends the blood into them too violent- 

 ly ; and it is thus, that the most part of sanguineous apoplexies are oc- 



