DEVELOPMENT OF REFLEX MECHANISMS 81 
THE MEDULLA OBLONGATA OF MAMMALS 
Here the arrangement of receptors and peripheral sensory 
nerve components is in all essential respects the same as in the 
urodele Amphibia. It is true that some details of the pattern 
have been altered—the lateral line system has vanished, the 
cochlea has been added to the internal ear, etc.—but the general 
morphological pattern is the same, so far as the peripheral neu- 
rones are concerned. But after the first synapse has been passed 
the sensory neurones of the second order also are found to be 
segregated into definite sensory centers, each related exclusively 
to one peripheral system—the general cutaneous nerves all termi- 
nate in the gelatinous substance of Rolando or other gray matter 
related exclusively to the nerves of this functional system; the 
visceral sensory and gustatory nerves all terminate in the nucleus 
of the fasciculus solitarius, whose neurones receive stimuli from 
no other important sources; the vestibular and cochlear nerves 
terminate in their respective nuclei. Each of these sensory 
centers constitutes a specific ‘nucleus,’ differentiated away from 
the primordial central gray, which in Amblystoma preserves its 
original generalized character to a large extent. The secondary 
pathways leading away from these sensory nuclei may in mammals 
be as specific functionally as are the peripheral nerve roots which 
enter them, the correlation of these elements into particular 
reflex systems being accomplished in other centers farther re- 
moved from the primary afferent neurones. 
Recurring to our analogy of a house with many doors, the 
medulla oblongata of larval Amblystoma may be compared with 
a building containing a large central hall, admittance to which 
may be gained from the outside through many doors; and leading 
out from. the main hall is a smaller number of short, wide corri- 
dors some of which lead directly to exits, others to a common 
living room in another part of the dwelling. The mammalian 
medulla oblongata, however, is more like a large apartment 
building, composed of many suites, each with its separate en- 
trance and exit, but the inner rooms of all having free communi- 
