138 THE STRUCTURE OF MAN 
form and size are evident to the naked eye, and are, I consider, 
to be interpreted as progressive. This is the lobus occipitalis 
of the cerebral hemisphere, in which we find great variation 
in the extent of the calcar avis, and the posterior cornu of 
the lateral ventricle. Exact statistics on this subject are a 
desideratum. 
{In connection with the question of structural degeneration of the brain, 
certain recent observations of Forsyth-Major are of especial interest. It has _ 
been generally assumed that the smooth cerebrum and exposed cerebellum of 
the Lemurs, which are placed at the root of the order Primates of which Man 
is the highest member, are primitive characters, indicative of a relationship 
with and origin from a lowly order of Mammals. Forsyth-Major has discovered 
evidence of structural simplification and degeneration, during Ontogeny, of 
the brain of certain Lemurs (apparently in correlation with preponderating 
development of the face and nose) which points to the conclusion that the 
supposed primitive characters named may be secondary and retrogressive— 
a welcome suggestion, in view of Cope’s discovery that the oldest known 
Lemurs (Anaptomorphide) had large and highly-organised brains. The 
brain of the human fcetus, at from three to five months, develops certain 
convolutions which are early lost and have nothing to do with those of the adult. 
Kolliker, Beer, Cunningham, and others have investigated them, and the 
latter, suggesting that they may be the expression of mechanical effects conse- 
quent on a “quadrupedal growth pause” in development, has proposed to 
term them “transitory fissures” (microgyri of Beer). Considerable interest 
attaches to the occasional appearance of convolutions upon the surface of the 
hemispheres in normally smooth-brained Mammals ; as also to the question 
whether these are progressive structures, or conversely, whether they, and 
the convolutions which seem to disappear during Ontogeny among the 
Lemuroidea, may have anything to do with the ‘transitory fissures” above- 
named. A wide field of inquiry is here opened up, which gives promise 
of most important results, ] ! 
PERIPHERAL NERVOUS SYSTEM 
But few retrogressive phenomena are here met with; among 
these are the present condition of the rami recurrentes of the three 
branches of the trigeminus and of the vagus, which run to the 
dura mater, and further, of the ramus auricularis of the latter nerve. 
The fact that in the region of the hypoglossus vestiges of 
the posterior roots with their ganglia have been found in human 
embryos, as they were long since in certain lower Mammals, 
indicates that assimilation of spinal or vertebral elements may 
be going on in the occipital region of the skull. Certain delicate 
nerve loops which lie in the region of the trigeminus, facialis 
1 (Cf. Forsyth-Major in Rothschild’s Novitates Zoologice, vol. i. p. 35 ; and Cun- 
ningham, Cunningham Memoirs, R. Irish Acad., No. VII. p. 30.] 
