238 ON THE STRUCTURE OF THE SKULL. 



proceed ; and, further forwards, the hemispheres of the cerebrum. 

 Anteriorly, both trabeculae reached as far as the anterior end 

 of the head, and here bent slightly upwards, so that they projected 

 a little into the frontal wall of the head, their ends lying in 

 front of the cerebrum. Almost at the end of each horn, how- 

 ever, I saw a small process, its immediate prolongation, pass 

 outwards and form, as it were, the nucleus for a small lateral 

 projection of the nasal process of the frontal wall. 



The middle trabecula grows, with the brain, further and 

 further into the cranial cavity ; and as the dura mater begins to 

 be now distinguishable, it becomes more readily obvious than 

 before, that the middle trabecula raises up a transverse fold of 

 it, which traverses the cranial cavity transversely.* The fold 

 itself passes laterally into the cranial wall ; it is highest in the 

 middle, where it encloses the median trabecula, and becomes 

 lower externally, where it forms, as it were, a short ala pro- 

 ceeding from the trabecula. With increasing elongation, the 

 trabecula becomes broader and broader towards its free end, and, 

 for a short time, its thickness increases. After this, however, it 

 gradually becomes thinner, without any change in its tissue, till, 

 at the end of the second period, it is only a thin lamella, and 

 after a short time (in the third period) entirely disappears. 



In Mammals, Birds, and Lizards, that is, in those animals in 

 general in which the middle cerebral vesicle is very strongly 

 bent up and forms a protuberance, while the base of the brain 

 exhibits a deep fold between the infundibulum and the posterior 

 cerebral vesicle, a similar part to this median trabecula of the 

 skull is found. 



In these animals, also, at a certain very early period of 

 embryonic life, it elevates a fold of the dura mater which passes 

 from one future petrous bone to the other, and after a certain 

 time projects strongly into the cranial cavity. Somewhat later, 

 however, it diminishes in height and thickness, as I have 

 especially observed in embryos of the Pig and Fowl, until at last 

 it disappears entirely in these higher animals also, the two layers 

 of the fold which it had raised up coming into contact. When 



* What Ratlike terms the "middle trabecula," appears to be only very indis- 

 tinetly developed in Fishes and Amphibia. 



