86 PEARL BRIGGS BULLARD 



is quite striking. This condition may be interpreted as a dorso- 

 ventral thickening and mesial fusion of the doi'sal horns (posterior 

 grey cokunns) since the gelatinous substance of Rolando and 

 the dorsal horn cells are present as normally. The spider mon- 

 key (fig. 17) possesses relatively short ventral or anterior horns 

 and this relative shortening is evident through all three regions 

 of the cord. 



In all the animals here studied, with one exception, the width 

 or thickness of the ventral horn in the cervical region exceeds 

 that of the dorsal horn. This exception is in the kangaroo in the 

 cervical region of which, the caput of the dorsal horn is wider 

 than is the ventral horn. This condition in the kangaroo is due, 

 not to a relatively extra wide dorsal horn but to a narrow ven- 

 tral horn. Because of its small anterior extremities, it is proba- 

 ble that cutaneous innervation may not have decreased to the 

 same extent as the innervation required by the muscles, which 

 receive motor or ventral root-fibers and which have atrophied 

 through greatly lessened use. If such be the case, the retained 

 width of the dorsal horn may be understood in that it contains 

 the cell bodies of association and commissural neurones about 

 which the dorsal root or sensory fibers terminate for purposes of 

 functionally associating different levels and the two sides of the 

 spinal cord with sensations brought into the cervical region. 



The lumbar region shows throughout the series the ventral 

 horn to be wider than the dorsal horn (columns 7 and 8, table 

 4). However, the average difference in the width of the two 

 horns in the lumbar region for the series is only 0.52 mm. while 

 in the cervical region the average difference is 0.64 mm., the 

 ventral horn being the wider. 



The difference in width of the dorsal and ventral horns is in 

 the thoracic region very much less than in either of the other 

 two regions (columns 7 and 8, table. 3). In most cases the dorsal 

 horn is somewhat the wider. This is what we should expect 

 since in the thoracic region the musculature is lessened in amount 

 to an extent greater than is the sensory area decreased. 



The dorso-ventral width of the grey commissure as taken 

 through the central canal (column 9, tables 2, 3, 4), varies through- 



