570 MANUAL OF HISTOLOGY. 



This would naturally allow of the distension with blood of the small 

 receptacles of the cavernous substance. Later Eckbart found in the dog 

 fibres running from the plexus ischiadicus to the hypogastricus, which lu> 

 showed to be the erection nerves. Loven found that during irritation of 

 these a bright red stream of blood spurted from a small arterial twig 

 suddenly on being opened ; the pressure of the blood, at the same time, 

 in the vessels of the penis continuing much less than in the carotid. Here, 

 then, we have before us a relaxation of the walls of the smaller arteries 

 brought about by stimulation of a nerve similar to that produced in the 

 heart by irritation of the vagus. 



But, besides this, no doubt hindrance to the exit of the blood from the 

 organ increases the erection. This is possibly brought about by the 

 m. transversus perinoei (Herile) preventing the return through the roots of 

 the penis. Also by the position of the venae profundce in the corpora 

 cavernosa, and the fact that the veins of the plexus pudendalis possess 

 numerous projections of smooth muscles. 



B. Organs of the Animal Group. 



6. Bony Apparatus. 



288. 



Although we have already referred at some length, in the second part 

 of our work, to the bony apparatus or osseous system in dealing with the 

 tissues of which bones are composed, there still remain some comple- 

 mentary considerations which must occupy us for a few moments. These 

 are, in the first place, the mode of connection of the various portions of 

 the skeleton with one another ; secondly, the vessels and nerves of bone ; 

 and thirdly, the substance with which the cavities of the latter filled up. 



The ways in which bones are joined together are, as is well known, very 

 various. While in the embryo the connecting masses are, in all proba- 

 bility, almost universally solid, but a small number of them remains so at 

 a later period. In such instances they are known to anatomists as 

 examples of synarthrosis, a mode of connection represented in sutures and 

 symphyses. In other rudimentary masses of this kind a process of liqui- 

 f action in the interior gives rise to the formation of cavities, while the 

 peripheral cells of the mass are transformed into the tissue of a capsule, 

 with its epithelial cells, &c. This mode of connection is designated as 

 diarthrosis, or jointed union. If, as is often the case with symphyses, 

 the process of liquifaction should cease at an early period, we have what 

 has been called half joints (Luschka). The latter are usually somewhat 

 ill-defined, and are variable in nature : no synovial capsule is to be recog- 

 nised in their interior. 



In regard now to the several media of articulation between bones, the 

 suture is united by what is incorrectly named suture-cartilage, which is 

 nothing less than a fine band of whitish fibrous connective-tissue. Sym- 

 physis is effected by hyaline, or fibrous cartilage and connective-tissue. 

 Here the ends of the bones are clothed with a layer of hyaline substance, 

 which, covered externally by connective-tissue, completes the union ; or 

 this cartilage passes gradually, more and more, into a fibrous mass, which 

 may at certain points give way to pure connective-tissue. We have already 

 referred to this texture, in speaking of fibrous cartilage, in 109, where the 

 intervertebral disks were fully described. The symphysis pubis and 

 sacroiliaca are half joints, as also almost invariably the points of union of 



