20 ON THE NERVOUS SYSTEM IV. 



Fig. . pig. 2. It is not easy, however, to demonstrate the presence of 



these last in recent specimens. They may be detected readily, 

 by making thin sections after the chord has been hardened in 

 . alcohol, and then treating them in the manner followed by Mr. 

 Clarke.* They are more conspicuous in the posterior horns of 

 the gray substance, where they form a well-defined deposit, 

 some of them being fusiform, others with three or more caudate appendages. After 

 the most careful examination, I have not detected any direct connection between 

 these caudate appendages and nerve tubes, and have not therefore been able to con- 

 firm the statement of Hanover and others, that nerve tubes originate or terminate in 

 nerve cells. The deposit of caudate cells just referred to seems to extend through 

 the whole length of the posterior horns of the chord ; they were not detected in the 

 anterior horns. 



On opening the chord from the back longitudinally, two white tracts or columns 

 are traced through its whole length and continued into the fourth ventricle, on the 

 floor of which they may be seen without dissection ; they do not, however, appear 

 to be any thing else, as will be seen hereafter, than the lower extremity of each 

 lateral crescent of white substance, as seen in Fig. 1. 



In the sections made and examined according to Mr. Clarke's method, I have al- 

 ways found a canal (Fig. 1, c) occupying nearly the centre of the chord; it is lined 

 by a well-defined layer of oval or columnar "epithelium cells, inclosing a distinct 

 cavity, which in all probability is filled with serum. This canal is continuous with 

 the cavity of the fourth ventricle. BischofF, Rokitansky, and others, seem to regard 

 the canal as of only temporary existence in the human body ; but Mr. Clarke's 

 observations show that it is constant, and it has been detached in many adult 

 animals ; it is therefore highly probable that something of the kind exists generally 

 in the Vertebrate division. Consequently, the opinion of Gall and Spurzheim, as 

 to the existence of a central canal, so long disputed, is fully confirmed. 



As already stated, the medulla oblongata, when seen in front or on the abdominal 

 side, appears to be a serial repetition of the crural and brachial enlargements. Ex- 

 amined on the back, the resemblance is less striking, there being a fourth ventricle 

 to which there is nothing similar in the other portions. If it has been more 

 generally described in connection with the encephalon, it has been rather from its 

 position in the cranial cavity, than from any marked peculiarity as regards structure, 

 or the kind of function of which it is the seat. When morphologically considered, 

 it is reducible to the common type of the chord, and the more complex condition 

 which it has in the higher animals, in consequence of the existence of the olives 

 and pyramids, is lost in the descending scale. 



The fourth ventricle is lined with a thin layer of gray substance, b (Fig. 3), and 

 this is covered by a layer of epithelium, c.^ The posterior pyramids (Plate I. Fig. 2, L) 



* This consists in hardening the preparation in alcohol, and then transferring to a mixture of one part 

 of acetic acid and three parts of alcohol, in which it is left for one or two hours ; it is then placed for 

 the same period in alcohol, and finally transferred to turpentine, which expels the alcohol and leaves 

 the whole transparent. 



