MYELON IN MAMMALIA. 75 



to the medulla oblongata, as it is likewise in the Cetacea. In most 

 Rodentia the myelon terminates in the lumbar region, but in the 

 rabbit it extends a little way into the sacrum. In the mouse the 

 relative proportion of the myelon to the brain is as 22 to 100. 



In the Cetacea and Sirenia, the myelon presents only the 

 anterior enlargement, which is very near the brain, and is remark- 

 able for the close aggregation of the origins of the nerves from 

 that part. The myelon is closely invested by the dura mater, which 

 is directly perforated by the nerves, and the sheath terminates 

 at the pointed end of the myelon, not being continued as such, 

 over the c cauda equina.' The myelon is small in proportion to 

 the size of the body, shows the central canal, and, Hunter 

 remarks, ' is more fibrous than in other animals ; when an attempt 

 is made to break it longitudinally, it tears with a fibrous ap- 

 pearance, but transversely it breaks irregularly.' * 



In the Elephant the dura mater surrounds the myelon less 

 closely than in the Cetacea, and the roots of the nerves have a 

 longer course within the sheath. In the Giraffe 2 I found the 

 myelon closely invested by the dura mater, which was thinner on 

 the dorsal than on the ventral side : it is chiefly remarkable for 

 the length of the cervical portion, which from the corpora pyra- 

 midalia to the pectoral or brachial enlargement measured four 

 feet three inches. The elongation of this part during foetal de- 

 velopement proceeding by uniform interstitial addition, the roots 

 of the nerves become equally separated from each other ; and, as 

 the lowest filament of one root was not further removed from the 

 hio-hest of the next below, than this from the succeedino; filament 



O J CD 



of the same root, such filaments were extended over an unusual 

 space of the myelon ; the root of the third cervical coming from a 

 tract of not less than six inches in length : the contrast between 

 the cervical myelon of the Porpoise and Giraffe in this respect is 

 striking. 



o 



With the singular exceptions of the Echidna, Hedgehog, and 

 certain bats, the mass of the myelon bears a direct ratio to that 

 of the body throughout the Mammalian series, and its structure 

 is essentially the same. In the adult human male it a little 

 exceeds an ounce in weight ; its tissue is firmer than that of the 



O 



brain. As in all Vertebrates, the ventral and dorsal surfaces are 

 respectively divided into equal moieties by a longitudinal fissure, 

 of which the dorsal is deepest, and, in the Mammalia, closest. In 

 Man, the interfissural plate of pia mater can be shown to be a 

 fold in the ventral (anterior) fissure, fig. 40, a, but is confluent as a 



1 xciv. p. 374. 2 xcvn'. 



