Vertebrata. 



331 



encephalon is always relatively small; its wall is only tliickened 

 below and at the sides, dorsally it is very thin ; a small process 

 extends from it to end above in a small body which varies considerably 

 in form, the pineal gland (epijDhysis) : ventrally, the wall is 

 evaginated to form a funnel-like pit, the infun- 

 d i b u 1 u m, with a peculiar appendage, primitively 

 an invagination of the epithelium of the mouth, 

 the pituitary body (hypophysis) . The 

 thickened lateral portions of the thalamen- 

 cephalon are termed optic thalami. The 

 cerebrum and the thalamenoephalon are separated 

 by a deep transverse fold dorsally. The mid- 

 brain has a thickened upper wall divided into 

 two halves (the optic lobes) by a longi- 

 tudinal furrow, and in the Mammalia is also 

 divided by a transverse groove, so that there 

 are four lobes, hence the name, corpora quad- 

 rigemina. The cerebellum, which is specially 

 well developed in Birds and Mammals, has, usually, 

 a much thickened upper wall, extending posteriorly 

 over the medulla, whose dorsal boundary is, on 

 the contrary, very thin, whilst in other respects 

 this portion of the brain is similar to the spinal 

 cord, into which it is continued posteriorly without 

 any sharp demarcation. The spinal cord 

 extends through the vertebral column, as an almost 

 cylindrical rod dwindling to a point posteriorly ; 

 at the two regions where the nerves for the limbs 

 originate, it is usually somewhat enlarged. Brain 

 and spinal cord may be seen even with the naked 

 eye to consist of two substances, the grey 

 matter and the white matter; the former 

 consists of very numerous ganglion cells, which 

 lie embedded in a peculiar kind of connective 

 tissue (neuroglia), the latter consists of nerve 

 fibrils. 



Often, e.g., in Mammalia, the primitive relations of the vei-tebral column and 

 spinal canal have undergone a change, for, ovping to the more rapid growth of 

 the former, the hind end of the spinal canal is left empty. The result of this 

 again is that the posterior spinal nei'ves run within the canal for some distance 

 before making their exit. 



The brain and spinal cord ai-e sm-ronnded by thi-ee connective tissue sheaths. 

 Most externally lies a fibrous, hai-d covering, the dura mater, which invests 

 the inner sui-face of the skull closely, whilst in the Mammalia, a special portion 

 of the periosteum lines the spinal canal; the dm-a mater frequently foi-ms 

 large folds stretching between the difEerent sections of the brain. Next to the 

 nervous matter lies a very vascular covering, the pia mater, and between 

 this and the dura is the thin arachnoid membrane; this last is not 



Pig. 273. Hoi-i- 

 zontal longitudinal 

 section through the 

 brain of a Vertebrate. 

 Diagrammatic. / 



cerebrum, me thala- 

 menoephalon, mi mid- 

 brain, b cerebellum, 

 e medulla, 1, 1 the 

 cavities of the cere- 

 bral hemisphere {ven- 

 triculi laterales), 3 

 cavity in the thala- 

 menoephalon {ven- 

 triculus ierfius), a 

 that of the mid-brain 

 {aqucsductits Sylvii), 

 4 that of the hind 

 brain (ventriculus 

 quarttis) . — Orig. 



