472 



THE TISSUES. 



the cord is richly supplied with plexuses of fine nerve-fibres which 

 accompany the vessels. At the base of the brain, many similar 

 plexuses occur on the arteries of the circle of Willis, which are 

 distributed in twigs ^^ of an inch in diameter through the entire 

 cerebral pia mater, following its vessels, but not those of the cere- 

 bellum. 



The vessels of the cord and encephalon themselves on leaving the 

 pia mater, are supplied to the gray matter much more abundantly 

 than to the white substance; the capillary plexus being closer (and 

 the capillaries themselves of less calibre), to which the dark color is 

 in part due. The interstices of the capillaries in the white sub- 

 stance are H ^ by gj of an inch. (E. Welter.) In the sheep's brain 

 the breadth of the interstices of the gray substance is three or four 

 times less than in the white. (Gerlach.) (Fig. 313.) The finest 

 capillaries in the cord measure ?4 \4 of an inch, and in the brain, 

 s o^o- of an inch. The manner in which the terminal arteries merge 

 into the capillaries on entering the gray matter of the convolutions 

 from the pia mater is shown by Fig. 314. 



Chemical Composition of the Nervous Centres. 



The composition of the elements of the nerve-fibres has already 

 been given, so far as it is understood (p. 430). The composition of' 

 the white and gray matter of the encephalon will now be specified. 



Yauquelin states that the spinal cord and medulla oblongata have 

 the same composition as the cerebrum, except that they contain 

 much more fat, with less albumen, ozmazome, and water. The ana- 

 lyses of the encephalon alone will be here given; and those of Yon 

 Bibra will be adopted as the most recent and reliable. 1 



The following is Yon Bibra's analysis of the gray and the white 

 matter, separately, of the brain of a man aged 30 years, who died 

 of phthisis: 



1 Comparative Investigations of the Brain of Man and the Mammalia. 

 heim, 1854. 



Mann- 



