226 MANUAL OF HISTOLOGY. 



envelopes, as those of the testicle, the kidneys, the spleen, the penis, and 

 clitoris. There is generally a considerable richness in elastic fibrillse here 

 also. This tissue may, besides, be continuous internally towards the 

 organ, with a banded or plaited network, in which smooth muscular 

 fibres appear sometimes as farther form-elements. This arrangement may 

 be seen in the cavernous portions of the urino-genital apparatus in the 

 lymphatic glands and spleen. 



b. The fasciae, which run externally into formless connective-tissue, and 

 likewise penetrate between the muscle fibres internally, in the form of 

 thin plates. At one time they have more the texture of tendon; at 

 another the elastic fibres gain the preponderance, to such an extent, in 

 certain cases, that abundant networks of the broadest fibres may occur. 



c. The perineurium, or, as it is usually called, the neurilemma, likewise 

 continuous externally with formless connective-tissue, and passing 

 inwards between the nervous fibres in a more homogeneous form. It 

 consists, on the larger nervous trunks, of regular bundles of connective- 

 tissue lying side by side, parallel with the course of the nerve. The 

 undulating arrangement of these it is which gives to the structure its 

 glistening plaited appearance. Beside these we find numerous elastic 

 fibres. The structure of the sheath continues thus down to the smaller 

 nervous branches, only decreasing in quantity ; here its substance loses 

 more and more the fibrillated character, and fusiform connective-tissue 

 corpuscles make their appearance at considerable intervals, until finally, 

 in the smallest ramifications, we find only a transparent homogeneous 

 membrane with single nuclei imbedded in it. There is, therefore, a 

 gradual transition here from a fully-developed connective or fibrous tissue 

 into a very plain connective-tissue substance. 



d. Periosteum and perichondrium. The first of these is a strong mem- 

 brane clothing the outer surface of bone, which, on account of its supplying 

 the bone with nutrition, is traversed by a multitude of blood-vessels. 

 Its external layers show a large proportion of connective-tissue, and its 

 internal, or those lying nearer the bone, more of the fine elastic fibrous 

 networks considered already. Its connection with the bone is effected by 

 means of the blood-vessels sinking from it into the latter, while, 

 externally, it merges into formless connective-tissue. Whenever processes 

 of mucous membrane extend over the surfaces of bony cavities, it is the 

 custom to speak of a coalescence of the former with the periosteum, 

 although this cannot be demonstrated. Perichondrium, except that it is 

 the enveloping membrane of cartilage, is similar in structure. It is rich 

 in blood-vessels, which are destined for the supply of the former ( 112). 

 In reticular cartilage we may remark the elastic fibres of its intermediate 

 substance passing continuously into similar elements of this connective- 

 tissue tunic. 



6. The serous membranes. In these, bundles of connective-tissue are 

 discovered intersecting each other in all directions, but they may appear as 

 though converted on the free surface into a more homogeneous layer. 

 Besides these, we encounter also, in tolerable abundance, and at times 

 even in large quantity, networks of fine elastic fibres in them. Here the 

 amount of blood-vessels is inconsiderable. Underneath, towards the 

 organ enclosed, this structure passes into a loose formless connective-tissue, 

 the so-called sub-serous, whilst the free surface is clothed with flattened 

 epithelium springing from the middle germinal plate ( 98). Theoretically, 

 the serous membranes were formerly held to be completely closed sacs, 



